vhost: Add polling mode

From: Razya Ladelsky <[email protected]>
Date: Thu, 31 Jul 2014 09:47:20 +0300
Subject: [PATCH] vhost: Add polling mode

When vhost is waiting for buffers from the guest driver (e.g., more packets to send in vhost-net's transmit queue), it normally goes to sleep and waits for the guest to "kick" it. This kick involves a PIO in the guest, and therefore an exit (and possibly userspace involvement in translating this PIO exit into a file descriptor event), all of which hurts performance.

If the system is under-utilized (has cpu time to spare), vhost can continuously poll the virtqueues for new buffers, and avoid asking the guest to kick us. This patch adds an optional polling mode to vhost, that can be enabled via a kernel module parameter, "poll_start_rate".

When polling is active for a virtqueue, the guest is asked to disable notification (kicks), and the worker thread continuously checks for new buffers. When it does discover new buffers, it simulates a "kick" by invoking the underlying backend driver (such as vhost-net), which thinks it got a real kick from the guest, and acts accordingly. If the underlying driver asks not to be kicked, we disable polling on this virtqueue.

We start polling on a virtqueue when we notice it has work to do. Polling on this virtqueue is later disabled after 3 seconds of polling turning up no new work, as in this case we are better off returning to the exit-based notification mechanism. The default timeout of 3 seconds can be changed with the "poll_stop_idle" kernel module parameter.

This polling approach makes lot of sense for new HW with posted-interrupts for which we have exitless host-to-guest notifications. But even with support for posted interrupts, guest-to-host communication still causes exits. Polling adds the missing part.

When systems are overloaded, there won't be enough cpu time for the various vhost threads to poll their guests' devices. For these scenarios, we plan to add support for vhost threads that can be shared by multiple devices, even of multiple vms.
Our ultimate goal is to implement the I/O acceleration features described in:
KVM Forum 2013: Efficient and Scalable Virtio (by Abel Gordon)
https://www.youtube.com/watch?v=9EyweibHfEs
and
https://www.mail-archive.com/[email protected]/msg98179.html

I ran some experiments with TCP stream netperf and filebench (having 2 threads performing random reads) benchmarks on an IBM System x3650 M4.
I have two machines, A and B. A hosts the vms, B runs the netserver. The vms (on A) run netperf, its destination server is running on B. All runs loaded the guests in a way that they were (cpu) saturated. For example, I ran netperf with 64B messages, which is heavily loading the vm (which is why its throughput is low).
The idea was to get it 100% loaded, so we can see that the polling is getting it to produce higher throughput.

The system had two cores per guest, as to allow for both the vcpu and the vhost thread to run concurrently for maximum throughput (but I didn't pin the threads to specific cores).
My experiments were fair in a sense that for both cases, with or without polling, I run both threads, vcpu and vhost, on 2 cores (set their affinity that way). The only difference was whether polling was enabled/disabled.

Results:

Netperf, 1 vm:
The polling patch improved throughput by ~33% (1516 MB/sec -> 2046 MB/sec). Number of exits/sec decreased 6x.
The same improvement was shown when I tested with 3 vms running netperf (4086 MB/sec -> 5545 MB/sec).

filebench, 1 vm:
ops/sec improved by 13% with the polling patch. Number of exits was reduced by 31%.
The same experiment with 3 vms running filebench showed similar numbers.

Signed-off-by Razya Ladelsky <[email protected]>

---
 drivers/vhost/net.c   |    6 +-
 drivers/vhost/scsi.c  |    6 +-
 drivers/vhost/vhost.c |  245 +++++++++++++++++++++++++++++++++++++++++++++++--
 drivers/vhost/vhost.h |   38 +++++++-
 4 files changed, 277 insertions(+), 18 deletions(-)

drivers/vhost/net.c +4 -2

index 971a760..558aecb 100644
@@ -742,8 +742,10 @@ static int vhost_net_open(struct inode *inode, struct file *f)
 	}
 	vhost_dev_init(dev, vqs, VHOST_NET_VQ_MAX);
 
-	vhost_poll_init(n->poll + VHOST_NET_VQ_TX, handle_tx_net, POLLOUT, dev);
-	vhost_poll_init(n->poll + VHOST_NET_VQ_RX, handle_rx_net, POLLIN, dev);
+	vhost_poll_init(n->poll + VHOST_NET_VQ_TX, handle_tx_net, POLLOUT,
+			vqs[VHOST_NET_VQ_TX]);
+	vhost_poll_init(n->poll + VHOST_NET_VQ_RX, handle_rx_net, POLLIN,
+			vqs[VHOST_NET_VQ_RX]);
 
 	f->private_data = n;
 

drivers/vhost/scsi.c +3 -3

index 4f4ffa4..665eeeb 100644
@@ -1528,9 +1528,9 @@ static int vhost_scsi_open(struct inode *inode, struct file *f)
 	if (!vqs)
 		goto err_vqs;
 
-	vhost_work_init(&vs->vs_completion_work, vhost_scsi_complete_cmd_work);
-	vhost_work_init(&vs->vs_event_work, tcm_vhost_evt_work);
-
+	vhost_work_init(&vs->vs_completion_work, NULL,
+						vhost_scsi_complete_cmd_work);
+	vhost_work_init(&vs->vs_event_work, NULL, tcm_vhost_evt_work);
 	vs->vs_events_nr = 0;
 	vs->vs_events_missed = false;
 

drivers/vhost/vhost.c +236 -9

index c90f437..fbe8174 100644
@@ -24,9 +24,17 @@
 #include <linux/slab.h>
 #include <linux/kthread.h>
 #include <linux/cgroup.h>
+#include <linux/jiffies.h>
 #include <linux/module.h>
 
 #include "vhost.h"
+static int poll_start_rate = 0;
+module_param(poll_start_rate, int, S_IRUGO|S_IWUSR);
+MODULE_PARM_DESC(poll_start_rate, "Start continuous polling of virtqueue when rate of events is at least this number per jiffy. If 0, never start polling.");
+
+static int poll_stop_idle = 3*HZ; /* 3 seconds */
+module_param(poll_stop_idle, int, S_IRUGO|S_IWUSR);
+MODULE_PARM_DESC(poll_stop_idle, "Stop continuous polling of virtqueue after this many jiffies of no work.");
 
 enum {
 	VHOST_MEMORY_MAX_NREGIONS = 64,

@@ -58,27 +66,28 @@ static int vhost_poll_wakeup(wait_queue_t *wait, unsigned mode, int sync,
 	return 0;
 }
 
-void vhost_work_init(struct vhost_work *work, vhost_work_fn_t fn)
+void vhost_work_init(struct vhost_work *work, struct vhost_virtqueue *vq,
+							vhost_work_fn_t fn)
 {
 	INIT_LIST_HEAD(&work->node);
 	work->fn = fn;
 	init_waitqueue_head(&work->done);
 	work->flushing = 0;
 	work->queue_seq = work->done_seq = 0;
+	work->vq = vq;
 }
 EXPORT_SYMBOL_GPL(vhost_work_init);
 
 /* Init poll structure */
 void vhost_poll_init(struct vhost_poll *poll, vhost_work_fn_t fn,
-		     unsigned long mask, struct vhost_dev *dev)
+		     unsigned long mask, struct vhost_virtqueue *vq)
 {
 	init_waitqueue_func_entry(&poll->wait, vhost_poll_wakeup);
 	init_poll_funcptr(&poll->table, vhost_poll_func);
 	poll->mask = mask;
-	poll->dev = dev;
+	poll->dev = vq->dev;
 	poll->wqh = NULL;
-
-	vhost_work_init(&poll->work, fn);
+	vhost_work_init(&poll->work, vq, fn);
 }
 EXPORT_SYMBOL_GPL(vhost_poll_init);
 

@@ -174,6 +183,86 @@ void vhost_poll_queue(struct vhost_poll *poll)
 }
 EXPORT_SYMBOL_GPL(vhost_poll_queue);
 
+/* Enable or disable virtqueue polling (vqpoll.enabled) for a virtqueue.
+ *
+ * Enabling this mode it tells the guest not to notify ("kick") us when its
+ * has made more work available on this virtqueue; Rather, we will continuously
+ * poll this virtqueue in the worker thread. If multiple virtqueues are polled,
+ * the worker thread polls them all, e.g., in a round-robin fashion.
+ * Note that vqpoll.enabled doesn't always mean that this virtqueue is
+ * actually being polled: The backend (e.g., net.c) may temporarily disable it
+ * using vhost_disable/enable_notify(), while vqpoll.enabled is unchanged.
+ *
+ * It is assumed that these functions are called relatively rarely, when vhost
+ * notices that this virtqueue's usage pattern significantly changed in a way
+ * that makes polling more efficient than notification, or vice versa.
+ * Also, we assume that vhost_vq_disable_vqpoll() is always called on vq
+ * cleanup, so any allocations done by vhost_vq_enable_vqpoll() can be
+ * reclaimed.
+ */
+static void vhost_vq_enable_vqpoll(struct vhost_virtqueue *vq)
+{
+	if (vq->vqpoll.enabled)
+		return; /* already enabled, nothing to do */
+	if (!vq->handle_kick)
+		return; /* polling will be a waste of time if no callback! */
+	if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY)) {
+		/* vq has guest notifications enabled. Disable them,
+		   and instead add vq to the polling list */
+		vhost_disable_notify(vq->dev, vq);
+		list_add_tail(&vq->vqpoll.link, &vq->dev->vqpoll_list);
+	}
+	vq->vqpoll.jiffies_last_kick = jiffies;
+	__get_user(vq->avail_idx, &vq->avail->idx);
+	vq->vqpoll.enabled = true;
+
+	/* Map userspace's vq->avail to the kernel's memory space. */
+	if (get_user_pages_fast((unsigned long)vq->avail, 1, 0,
+		&vq->vqpoll.avail_page) != 1) {
+		/* TODO: can this happen, as we check access
+		to vq->avail in advance? */
+		BUG();
+	}
+	vq->vqpoll.avail_mapped = (struct vring_avail *) (
+		(unsigned long)kmap(vq->vqpoll.avail_page) |
+		((unsigned long)vq->avail & ~PAGE_MASK));
+}
+
+/*
+ * This function doesn't always succeed in changing the mode. Sometimes
+ * a temporary race condition prevents turning on guest notifications, so
+ * vq should be polled next time again.
+ */
+static void vhost_vq_disable_vqpoll(struct vhost_virtqueue *vq)
+{
+	if (!vq->vqpoll.enabled)
+		return; /* already disabled, nothing to do */
+
+	vq->vqpoll.enabled = false;
+
+	if (!list_empty(&vq->vqpoll.link)) {
+		/* vq is on the polling list, remove it from this list and
+		 * instead enable guest notifications. */
+		list_del_init(&vq->vqpoll.link);
+		if (unlikely(vhost_enable_notify(vq->dev, vq))
+			&& !vq->vqpoll.shutdown) {
+			/* Race condition: guest wrote before we enabled
+			 * notification, so we'll never get a notification for
+			 * this work - so continue polling mode for a while. */
+			vhost_disable_notify(vq->dev, vq);
+			vq->vqpoll.enabled = true;
+			vhost_enable_notify(vq->dev, vq);
+			return;
+		}
+	}
+
+	if (vq->vqpoll.avail_mapped) {
+		kunmap(vq->vqpoll.avail_page);
+		put_page(vq->vqpoll.avail_page);
+		vq->vqpoll.avail_mapped = 0;
+	}
+}
+
 static void vhost_vq_reset(struct vhost_dev *dev,
 			   struct vhost_virtqueue *vq)
 {

@@ -199,6 +288,48 @@ static void vhost_vq_reset(struct vhost_dev *dev,
 	vq->call = NULL;
 	vq->log_ctx = NULL;
 	vq->memory = NULL;
+	INIT_LIST_HEAD(&vq->vqpoll.link);
+	vq->vqpoll.enabled = false;
+	vq->vqpoll.shutdown = false;
+	vq->vqpoll.avail_mapped = NULL;
+}
+
+/* roundrobin_poll() takes worker->vqpoll_list, and returns one of the
+ * virtqueues which the caller should kick, or NULL in case none should be
+ * kicked. roundrobin_poll() also disables polling on a virtqueue which has
+ * been polled for too long without success.
+ *
+ * This current implementation (the "round-robin" implementation) only
+ * polls the first vq in the list, returning it or NULL as appropriate, and
+ * moves this vq to the end of the list, so next time a different one is
+ * polled.
+ */
+static struct vhost_virtqueue *roundrobin_poll(struct list_head *list)
+{
+	struct vhost_virtqueue *vq;
+	u16 avail_idx;
+
+	if (list_empty(list))
+		return NULL;
+
+	vq = list_first_entry(list, struct vhost_virtqueue, vqpoll.link);
+	WARN_ON(!vq->vqpoll.enabled);
+	list_move_tail(&vq->vqpoll.link, list);
+
+	/* See if there is any new work available from the guest. */
+	/* TODO: can check the optional idx feature, and if we haven't
+	* reached that idx yet, don't kick... */
+	avail_idx = vq->vqpoll.avail_mapped->idx;
+	if (avail_idx != vq->last_avail_idx)
+		return vq;
+
+	if (jiffies > vq->vqpoll.jiffies_last_kick + poll_stop_idle) {
+		/* We've been polling this virtqueue for a long time with no
+		* results, so switch back to guest notification
+		*/
+		vhost_vq_disable_vqpoll(vq);
+	}
+	return NULL;
 }
 
 static int vhost_worker(void *data)

@@ -237,12 +368,62 @@ static int vhost_worker(void *data)
 		spin_unlock_irq(&dev->work_lock);
 
 		if (work) {
+			struct vhost_virtqueue *vq = work->vq;
 			__set_current_state(TASK_RUNNING);
 			work->fn(work);
+			/* Keep track of the work rate, for deciding when to
+			 * enable polling */
+			if (vq) {
+				if (vq->vqpoll.jiffies_last_work != jiffies) {
+					vq->vqpoll.jiffies_last_work = jiffies;
+					vq->vqpoll.work_this_jiffy = 0;
+				}
+				vq->vqpoll.work_this_jiffy++;
+			}
+			/* If vq is in the round-robin list of virtqueues being
+			 * constantly checked by this thread, move vq the end
+			 * of the queue, because it had its fair chance now.
+			 */
+			if (vq && !list_empty(&vq->vqpoll.link)) {
+				list_move_tail(&vq->vqpoll.link,
+					&dev->vqpoll_list);
+			}
+			/* Otherwise, if this vq is looking for notifications
+			 * but vq polling is not enabled for it, do it now.
+			 */
+			else if (poll_start_rate && vq && vq->handle_kick &&
+				!vq->vqpoll.enabled &&
+				!vq->vqpoll.shutdown &&
+				!(vq->used_flags & VRING_USED_F_NO_NOTIFY) &&
+				vq->vqpoll.work_this_jiffy >=
+					poll_start_rate) {
+				vhost_vq_enable_vqpoll(vq);
+			}
+		}
+		/* Check one virtqueue from the round-robin list */
+		if (!list_empty(&dev->vqpoll_list)) {
+			struct vhost_virtqueue *vq;
+
+			vq = roundrobin_poll(&dev->vqpoll_list);
+
+			if (vq) {
+				vq->handle_kick(&vq->poll.work);
+				vq->vqpoll.jiffies_last_kick = jiffies;
+			}
+
+			/* If our polling list isn't empty, ask to continue
+			 * running this thread, don't yield.
+			 */
+			__set_current_state(TASK_RUNNING);
 			if (need_resched())
 				schedule();
-		} else
-			schedule();
+		} else {
+			if (work) {
+				if (need_resched())
+					schedule();
+			} else
+				schedule();
+		}
 
 	}
 	unuse_mm(dev->mm);

@@ -306,6 +487,7 @@ void vhost_dev_init(struct vhost_dev *dev,
 	dev->mm = NULL;
 	spin_lock_init(&dev->work_lock);
 	INIT_LIST_HEAD(&dev->work_list);
+	INIT_LIST_HEAD(&dev->vqpoll_list);
 	dev->worker = NULL;
 
 	for (i = 0; i < dev->nvqs; ++i) {

@@ -318,7 +500,7 @@ void vhost_dev_init(struct vhost_dev *dev,
 		vhost_vq_reset(dev, vq);
 		if (vq->handle_kick)
 			vhost_poll_init(&vq->poll, vq->handle_kick,
-					POLLIN, dev);
+					POLLIN, vq);
 	}
 }
 EXPORT_SYMBOL_GPL(vhost_dev_init);

@@ -350,7 +532,7 @@ static int vhost_attach_cgroups(struct vhost_dev *dev)
 	struct vhost_attach_cgroups_struct attach;
 
 	attach.owner = current;
-	vhost_work_init(&attach.work, vhost_attach_cgroups_work);
+	vhost_work_init(&attach.work, NULL, vhost_attach_cgroups_work);
 	vhost_work_queue(dev, &attach.work);
 	vhost_work_flush(dev, &attach.work);
 	return attach.ret;

@@ -444,6 +626,26 @@ void vhost_dev_stop(struct vhost_dev *dev)
 }
 EXPORT_SYMBOL_GPL(vhost_dev_stop);
 
+/* shutdown_vqpoll() asks the worker thread to shut down virtqueue polling
+ * mode for a given virtqueue which is itself being shut down. We ask the
+ * worker thread to do this rather than doing it directly, so that we don't
+ * race with the worker thread's use of the queue.
+ */
+static void shutdown_vqpoll_work(struct vhost_work *work)
+{
+	work->vq->vqpoll.shutdown = true;
+	vhost_vq_disable_vqpoll(work->vq);
+	WARN_ON(work->vq->vqpoll.avail_mapped);
+}
+
+static void shutdown_vqpoll(struct vhost_virtqueue *vq)
+{
+	struct vhost_work work;
+
+	vhost_work_init(&work, vq, shutdown_vqpoll_work);
+	vhost_work_queue(vq->dev, &work);
+	vhost_work_flush(vq->dev, &work);
+}
 /* Caller should have device mutex if and only if locked is set */
 void vhost_dev_cleanup(struct vhost_dev *dev, bool locked)
 {

@@ -460,6 +662,7 @@ void vhost_dev_cleanup(struct vhost_dev *dev, bool locked)
 			eventfd_ctx_put(dev->vqs[i]->call_ctx);
 		if (dev->vqs[i]->call)
 			fput(dev->vqs[i]->call);
+		shutdown_vqpoll(dev->vqs[i]);
 		vhost_vq_reset(dev, dev->vqs[i]);
 	}
 	vhost_dev_free_iovecs(dev);

@@ -1491,6 +1694,19 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
 	u16 avail_idx;
 	int r;
 
+	/* In polling mode, when the backend (e.g., net.c) asks to enable
+	 * notifications, we don't enable guest notifications. Instead, start
+	 * polling on this vq by adding it to the round-robin list.
+	 */
+	if (vq->vqpoll.enabled) {
+		if (list_empty(&vq->vqpoll.link)) {
+			list_add_tail(&vq->vqpoll.link,
+				&vq->dev->vqpoll_list);
+			vq->vqpoll.jiffies_last_kick = jiffies;
+		}
+		return false;
+	}
+
 	if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY))
 		return false;
 	vq->used_flags &= ~VRING_USED_F_NO_NOTIFY;

@@ -1528,6 +1744,17 @@ void vhost_disable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
 {
 	int r;
 
+	/* If this virtqueue is vqpoll.enabled, and on the polling list, it
+	 * will generate notifications even if the guest is asked not to send
+	 * them. So we must remove it from the round-robin polling list.
+	 * Note that vqpoll.enabled remains set.
+	 */
+	if (vq->vqpoll.enabled) {
+		if (!list_empty(&vq->vqpoll.link))
+			list_del_init(&vq->vqpoll.link);
+		return;
+	}
+
 	if (vq->used_flags & VRING_USED_F_NO_NOTIFY)
 		return;
 	vq->used_flags |= VRING_USED_F_NO_NOTIFY;

drivers/vhost/vhost.h +34 -4

index 3eda654..11aaaf4 100644
@@ -24,6 +24,7 @@ struct vhost_work {
 	int			  flushing;
 	unsigned		  queue_seq;
 	unsigned		  done_seq;
+	struct vhost_virtqueue    *vq;
 };
 
 /* Poll a file (eventfd or socket) */

@@ -37,11 +38,12 @@ struct vhost_poll {
 	struct vhost_dev	 *dev;
 };
 
-void vhost_work_init(struct vhost_work *work, vhost_work_fn_t fn);
+void vhost_work_init(struct vhost_work *work, struct vhost_virtqueue *vq,
+							vhost_work_fn_t fn);
 void vhost_work_queue(struct vhost_dev *dev, struct vhost_work *work);
 
 void vhost_poll_init(struct vhost_poll *poll, vhost_work_fn_t fn,
-		     unsigned long mask, struct vhost_dev *dev);
+		     unsigned long mask, struct vhost_virtqueue  *vq);
 int vhost_poll_start(struct vhost_poll *poll, struct file *file);
 void vhost_poll_stop(struct vhost_poll *poll);
 void vhost_poll_flush(struct vhost_poll *poll);

@@ -54,8 +56,6 @@ struct vhost_log {
 	u64 len;
 };
 
-struct vhost_virtqueue;
-
 /* The virtqueue structure describes a queue attached to a device. */
 struct vhost_virtqueue {
 	struct vhost_dev *dev;

@@ -110,6 +110,35 @@ struct vhost_virtqueue {
 	/* Log write descriptors */
 	void __user *log_base;
 	struct vhost_log *log;
+	struct {
+      /* When a virtqueue is in vqpoll.enabled mode, it declares
+       * that instead of using guest notifications (kicks) to
+       * discover new work, we prefer to continuously poll this
+       * virtqueue in the worker thread.
+       * If !enabled, the rest of the fields below are undefined.
+       */
+		bool enabled;
+      /* vqpoll.enabled doesn't always mean that this virtqueue is
+       * actually being polled: The backend (e.g., net.c) may
+       * temporarily disable it using vhost_disable/enable_notify().
+       * vqpoll.link is used to maintain the thread's round-robin
+       * list of virtqueues that actually need to be polled.
+       * Note list_empty(link) means this virtqueue isn't polled.
+       */
+		struct list_head link;
+      /* If this flag is true, the virtqueue is being shut down,
+       * so vqpoll should not be re-enabled.
+       */
+		bool shutdown;
+      /* Various counters used to decide when to enter polling mode
+       * or leave it and return to notification mode.
+       */
+		unsigned long jiffies_last_kick;
+		unsigned long jiffies_last_work;
+		int work_this_jiffy;
+		struct page *avail_page;
+		volatile struct vring_avail *avail_mapped;
+	} vqpoll;
 };
 
 struct vhost_dev {

@@ -123,6 +152,7 @@ struct vhost_dev {
 	spinlock_t work_lock;
 	struct list_head work_list;
 	struct task_struct *worker;
+	struct list_head vqpoll_list;
 };
 
 void vhost_dev_init(struct vhost_dev *, struct vhost_virtqueue **vqs, int nvqs);

Signature · 1 line

1.7.9.5

And, did your tests actually produce 100% load on both host CPUs?

From: "Michael S. Tsirkin" <[email protected]>
Date: Sun, 10 Aug 2014 21:45:59 +0200

Quoted 1 line · Razya Ladelsky

...

Quoted 1 line

And, did your tests actually produce 100% load on both host CPUs?

...

Michael, please do not quote an entire patch just to ask a one line question.

I truly, truly, wish it was simpler in modern email clients to delete the unrelated quoted material because I bet when people do this they are simply being lazy.

Thank you.

From: "Michael S. Tsirkin" <[email protected]>
To: David Miller <[email protected]>
Cc: Razya Ladelsky/Haifa/IBM@IBMIL, [email protected], Alex Glikson/Haifa/IBM@IBMIL, Eran Raichstein/Haifa/IBM@IBMIL, Yossi Kuperman1/Haifa/IBM@IBMIL, Joel Nider/Haifa/IBM@IBMIL, [email protected], [email protected], [email protected], [email protected]
Date: 12/08/2014 12:18 PM
Subject: Re: [PATCH] vhost: Add polling mode

Quoted 8 lines · David Miller

From: "Michael S. Tsirkin" <[email protected]>
Date: Sun, 10 Aug 2014 21:45:59 +0200

Quoted 1 line · Razya Ladelsky

...

Quoted 1 line

And, did your tests actually produce 100% load on both host CPUs?

...

Quoted 8 lines

Michael, please do not quote an entire patch just to ask a one line question.

I truly, truly, wish it was simpler in modern email clients to delete the unrelated quoted material because I bet when people do this they are simply being lazy.

Thank you.

Lazy - mea culpa, though I'm using mutt so it isn't even hard.

The question still stands: the test results are only valid if CPU was at 100% in all configurations.
This is the reason I generally prefer it when people report throughput divided by CPU (power would be good too but it still isn't easy for people to get that number).

Signature · 1 line

MST

"Michael S. Tsirkin" <[email protected]> wrote on 12/08/2014 12:18:50 PM:

Quoted 28 lines

From: "Michael S. Tsirkin" <[email protected]>
To: David Miller <[email protected]>
Cc: Razya Ladelsky/Haifa/IBM@IBMIL, [email protected], Alex Glikson/Haifa/IBM@IBMIL, Eran Raichstein/Haifa/IBM@IBMIL, Yossi Kuperman1/Haifa/IBM@IBMIL, Joel Nider/Haifa/IBM@IBMIL, [email protected], [email protected], [email protected], [email protected]
Date: 12/08/2014 12:18 PM
Subject: Re: [PATCH] vhost: Add polling mode

Quoted 8 lines · David Miller

From: "Michael S. Tsirkin" <[email protected]>
Date: Sun, 10 Aug 2014 21:45:59 +0200

Quoted 1 line · Razya Ladelsky

...

Quoted 1 line

And, did your tests actually produce 100% load on both host CPUs?

...

Quoted 8 lines

Michael, please do not quote an entire patch just to ask a one line question.

I truly, truly, wish it was simpler in modern email clients to delete the unrelated quoted material because I bet when people do this they are simply being lazy.

Thank you.

Quoted 8 lines

Lazy - mea culpa, though I'm using mutt so it isn't even hard.

The question still stands: the test results are only valid if CPU was at 100% in all configurations.
This is the reason I generally prefer it when people report throughput divided by CPU (power would be good too but it still isn't easy for people to get that number).

Hi Michael,

Sorry for the delay, had some problems with my mailbox, and I realized just now that
my reply wasn't sent.
The vm indeed ALWAYS utilized 100% cpu, whether polling was enabled or not.
The vhost thread utilized less than 100% (of the other cpu) when polling was disabled.
Enabling polling increased its utilization to 100% (in which case both cpus were 100% utilized).

Quoted 3 lines

Signature · 1 line

MST

Quoted 11 lines

Hi Michael,

Sorry for the delay, had some problems with my mailbox, and I realized

just now that
my reply wasn't sent.
The vm indeed ALWAYS utilized 100% cpu, whether polling was enabled or

not.
The vhost thread utilized less than 100% (of the other cpu) when

Quoted 4 lines

was disabled.
Enabling polling increased its utilization to 100% (in which case both

cpus were 100% utilized).

Hmm this means the testing wasn't successful then, as you said:

The idea was to get it 100% loaded, so we can see that the polling is getting it to produce higher throughput.

in fact here you are producing more throughput but spending more power to produce it, which can have any number of explanations besides polling improving the efficiency. For example, increasing system load might disable host power management.

Quoted 4 lines

Quoted 4 lines

Signature · 2 lines

-- 
MST

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Quoted 11 lines

Quoted 11 lines

Hi Michael,

Sorry for the delay, had some problems with my mailbox, and I realized

just now that
my reply wasn't sent.
The vm indeed ALWAYS utilized 100% cpu, whether polling was enabled or

not.
The vhost thread utilized less than 100% (of the other cpu) when

polling

Quoted 7 lines

Quoted 4 lines

was disabled.
Enabling polling increased its utilization to 100% (in which case both

cpus were 100% utilized).

Hmm this means the testing wasn't successful then, as you said:

Quoted 8 lines

The idea was to get it 100% loaded, so we can see that the polling is getting it to produce higher throughput.

in fact here you are producing more throughput but spending more power to produce it, which can have any number of explanations besides polling improving the efficiency. For example, increasing system load might disable host power management.

Hi Michael,
I re-ran the tests, this time with the "turbo mode" and "C-states" features off.
No Polling:
1 VM running netperf (msg size 64B): 1107 Mbits/sec
Polling:
1 VM running netperf (msg size 64B): 1572 Mbits/sec

As you can see from the new results, the numbers are lower, but relatively (polling on/off) there's no change. Thank you,
Razya

Quoted 14 lines

Quoted 9 lines

Quoted 9 lines

Signature · 2 lines

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That was just one example. There many other possibilities. Either actually make the systems load all host CPUs equally, or divide throughput by host CPU.

Quoted 4 lines

That was just one example. There many other possibilities. Either actually make the systems load all host CPUs equally, or divide throughput by host CPU.

The polling patch adds this capability to vhost, reducing costly exit overhead when the vm is loaded.

In order to load the vm I ran netperf with msg size of 256:

Without polling: 2480 Mbits/sec, utilization: vm - 100% vhost - 64%
With Polling: 4160 Mbits/sec, utilization: vm - 100% vhost - 100%

Therefore, throughput/cpu without polling is 15.1, and 20.8 with polling.

My intention was to load vhost as close as possible to 100% utilization without polling, in order to compare it to the polling utilization case (where vhost is always 100%).
The best use case, of course, would be when the shared vhost thread work (TBD) is integrated and then vhost will actually be using its polling cycles to handle requests of multiple devices (even from multiple vms).

Thanks,
Razya

From: "Michael S. Tsirkin" <[email protected]>
To: Razya Ladelsky/Haifa/IBM@IBMIL,
Cc: [email protected], Alex Glikson/Haifa/IBM@IBMIL, Eran Raichstein/Haifa/IBM@IBMIL, Yossi Kuperman1/Haifa/IBM@IBMIL, Joel Nider/Haifa/IBM@IBMIL, [email protected], linux- [email protected], [email protected], [email protected]
Date: 10/08/2014 10:45 PM
Subject: Re: [PATCH] vhost: Add polling mode

Quoted 4 lines · Razya Ladelsky

From: Razya Ladelsky <[email protected]>
Date: Thu, 31 Jul 2014 09:47:20 +0300
Subject: [PATCH] vhost: Add polling mode

Quoted 1 line

When vhost is waiting for buffers from the guest driver (e.g.,

more packets to

Quoted 1 line

send in vhost-net's transmit queue), it normally goes to sleep and

waits for the

Quoted 1 line

guest to "kick" it. This kick involves a PIO in the guest, and

therefore an exit

Quoted 1 line

(and possibly userspace involvement in translating this PIO exit into

Quoted 3 lines

descriptor event), all of which hurts performance.

If the system is under-utilized (has cpu time to spare), vhost can

continuously

Quoted 1 line

poll the virtqueues for new buffers, and avoid asking the guest to

Quoted 1 line

This patch adds an optional polling mode to vhost, that can be enabled

Quoted 4 lines

kernel module parameter, "poll_start_rate".

When polling is active for a virtqueue, the guest is asked to disable notification (kicks), and the worker thread continuously checks

for new buffers.

Quoted 1 line

When it does discover new buffers, it simulates a "kick" by invoking

Quoted 1 line

underlying backend driver (such as vhost-net), which thinks it got

a real kick

Quoted 1 line

from the guest, and acts accordingly. If the underlying driver

asks not to be

Quoted 3 lines

kicked, we disable polling on this virtqueue.

We start polling on a virtqueue when we notice it has work to do.

Quoted 1 line

this virtqueue is later disabled after 3 seconds of polling

turning up no new

Quoted 1 line

work, as in this case we are better off returning to the exit-

based notification

Quoted 3 lines

mechanism. The default timeout of 3 seconds can be changed with the "poll_stop_idle" kernel module parameter.

Quoted 1 line

This polling approach makes lot of sense for new HW with posted-

interrupts for

Quoted 1 line

which we have exitless host-to-guest notifications. But even with

support for

Quoted 1 line

posted interrupts, guest-to-host communication still causes exits.

Polling adds

Quoted 3 lines

the missing part.

When systems are overloaded, there won't be enough cpu time for the

Quoted 1 line

vhost threads to poll their guests' devices. For these scenarios,

we plan to add

Quoted 1 line

support for vhost threads that can be shared by multiple devices, even

Quoted 2 lines

multiple vms.
Our ultimate goal is to implement the I/O acceleration features

described in:

Quoted 6 lines

KVM Forum 2013: Efficient and Scalable Virtio (by Abel Gordon)
https://www.youtube.com/watch?v=9EyweibHfEs
and
https://www.mail-archive.com/[email protected]/msg98179.html

I ran some experiments with TCP stream netperf and filebench

(having 2 threads

Quoted 4 lines

performing random reads) benchmarks on an IBM System x3650 M4. I have two machines, A and B. A hosts the vms, B runs the netserver. The vms (on A) run netperf, its destination server is running on B. All runs loaded the guests in a way that they were (cpu)

saturated. For example,

Quoted 1 line

I ran netperf with 64B messages, which is heavily loading the vm

(which is why

Quoted 2 lines

its throughput is low).
The idea was to get it 100% loaded, so we can see that the polling

is getting it

Quoted 1 line

to produce higher throughput.

And, did your tests actually produce 100% load on both host CPUs?

Quoted 1 line

The system had two cores per guest, as to allow for both the vcpu

and the vhost

Quoted 1 line

thread to run concurrently for maximum throughput (but I didn't

pin the threads

Quoted 2 lines

to specific cores).
My experiments were fair in a sense that for both cases, with or

Quoted 1 line

polling, I run both threads, vcpu and vhost, on 2 cores (set their

affinity that

Quoted 6 lines

way). The only difference was whether polling was enabled/disabled.

Results:

Netperf, 1 vm:
The polling patch improved throughput by ~33% (1516 MB/sec -> 2046

Quoted 2 lines

Number of exits/sec decreased 6x.
The same improvement was shown when I tested with 3 vms running

Quoted 4 lines

(4086 MB/sec -> 5545 MB/sec).

filebench, 1 vm:
ops/sec improved by 13% with the polling patch. Number of exits

was reduced by

Quoted 2 lines

31%.
The same experiment with 3 vms running filebench showed similar

Quoted 6 lines

Signed-off-by Razya Ladelsky <[email protected]>

---
 drivers/vhost/net.c   |    6 +-
 drivers/vhost/scsi.c  |    6 +-
 drivers/vhost/vhost.c |  245 ++++++++++++++++++++++++++++++++++++

+++++++++++--

Quoted 3 lines

 drivers/vhost/vhost.h |   38 +++++++-
 4 files changed, 277 insertions(+), 18 deletions(-)

drivers/vhost/net.c static int vhost_net_open(struct inode

Quoted 5 lines

drivers/vhost/net.c +0 -0

index 971a760..558aecb 100644
@@ -742,8 +742,10 @@ static int vhost_net_open(struct inode 

*inode, struct file *f)

Quoted 4 lines

drivers/vhost/net.c +0 -1

    }
    vhost_dev_init(dev, vqs, VHOST_NET_VQ_MAX);
 
-   vhost_poll_init(n->poll + VHOST_NET_VQ_TX, handle_tx_net, POLLOUT, 

drivers/vhost/net.c static int vhost_net_open(struct inode · line 746

Quoted 1 line

drivers/vhost/net.c +0 -1

-   vhost_poll_init(n->poll + VHOST_NET_VQ_RX, handle_rx_net, POLLIN, 

Quoted 5 lines

drivers/vhost/net.c +4 -0

+   vhost_poll_init(n->poll + VHOST_NET_VQ_TX, handle_tx_net, POLLOUT,
+         vqs[VHOST_NET_VQ_TX]);
+   vhost_poll_init(n->poll + VHOST_NET_VQ_RX, handle_rx_net, POLLIN,
+         vqs[VHOST_NET_VQ_RX]);
 

drivers/vhost/scsi.c static int vhost_scsi_open(struct inode

Quoted 7 lines

drivers/vhost/net.c +0 -0

    f->private_data = n;
 

drivers/vhost/scsi.c +0 -0

index 4f4ffa4..665eeeb 100644
@@ -1528,9 +1528,9 @@ static int vhost_scsi_open(struct inode 

*inode, struct file *f)

Quoted 4 lines

drivers/vhost/scsi.c +0 -1

    if (!vqs)
       goto err_vqs;
 
-   vhost_work_init(&vs->vs_completion_work, 

Quoted 13 lines

drivers/vhost/scsi.c +3 -2

-   vhost_work_init(&vs->vs_event_work, tcm_vhost_evt_work);
-
+   vhost_work_init(&vs->vs_completion_work, NULL,
+                  vhost_scsi_complete_cmd_work);
+   vhost_work_init(&vs->vs_event_work, NULL, tcm_vhost_evt_work);
    vs->vs_events_nr = 0;
    vs->vs_events_missed = false;
 

drivers/vhost/vhost.c +0 -0

index c90f437..fbe8174 100644

drivers/vhost/vhost.c line 33

Quoted 10 lines

drivers/vhost/vhost.c +4 -0

@@ -24,9 +24,17 @@
 #include <linux/slab.h>
 #include <linux/kthread.h>
 #include <linux/cgroup.h>
+#include <linux/jiffies.h>
 #include <linux/module.h>
 
 #include "vhost.h"
+static int poll_start_rate = 0;
+module_param(poll_start_rate, int, S_IRUGO|S_IWUSR);
+MODULE_PARM_DESC(poll_start_rate, "Start continuous polling of 

virtqueue when rate of events is at least this number per jiffy. If 0, never start polling.");

drivers/vhost/vhost.c

Quoted 4 lines

drivers/vhost/vhost.c +4 -0

+
+static int poll_stop_idle = 3*HZ; /* 3 seconds */
+module_param(poll_stop_idle, int, S_IRUGO|S_IWUSR);
+MODULE_PARM_DESC(poll_stop_idle, "Stop continuous polling of 

virtqueue after this many jiffies of no work.");

drivers/vhost/vhost.c static int vhost_poll_wakeup(wait_queue_t

Quoted 4 lines

drivers/vhost/vhost.c +0 -0

 enum {
    VHOST_MEMORY_MAX_NREGIONS = 64,

@@ -58,27 +66,28 @@ static int vhost_poll_wakeup(wait_queue_t 

*wait, unsigned mode, int sync,

drivers/vhost/vhost.c static int vhost_poll_wakeup(wait_queue_t *wait, unsigned mode, int sync,

Quoted 5 lines

drivers/vhost/vhost.c +1 -1

    return 0;
 }
 
-void vhost_work_init(struct vhost_work *work, vhost_work_fn_t fn)
+void vhost_work_init(struct vhost_work *work, struct vhost_virtqueue 

Quoted 28 lines

drivers/vhost/vhost.c +5 -4

+                     vhost_work_fn_t fn)
 {
    INIT_LIST_HEAD(&work->node);
    work->fn = fn;
    init_waitqueue_head(&work->done);
    work->flushing = 0;
    work->queue_seq = work->done_seq = 0;
+   work->vq = vq;
 }
 EXPORT_SYMBOL_GPL(vhost_work_init);
 
 /* Init poll structure */
 void vhost_poll_init(struct vhost_poll *poll, vhost_work_fn_t fn,
-           unsigned long mask, struct vhost_dev *dev)
+           unsigned long mask, struct vhost_virtqueue *vq)
 {
    init_waitqueue_func_entry(&poll->wait, vhost_poll_wakeup);
    init_poll_funcptr(&poll->table, vhost_poll_func);
    poll->mask = mask;
-   poll->dev = dev;
+   poll->dev = vq->dev;
    poll->wqh = NULL;
-
-   vhost_work_init(&poll->work, fn);
+   vhost_work_init(&poll->work, vq, fn);
 }
 EXPORT_SYMBOL_GPL(vhost_poll_init);
 

drivers/vhost/vhost.c void vhost_poll_queue(struct vhost_poll *poll) · line 186

Quoted 5 lines

drivers/vhost/vhost.c +1 -0

@@ -174,6 +183,86 @@ void vhost_poll_queue(struct vhost_poll *poll)
 }
 EXPORT_SYMBOL_GPL(vhost_poll_queue);
 
+/* Enable or disable virtqueue polling (vqpoll.enabled) for a 

drivers/vhost/vhost.c void vhost_poll_queue(struct vhost_poll *poll) · line 188

Quoted 2 lines

drivers/vhost/vhost.c +2 -0

+ *
+ * Enabling this mode it tells the guest not to notify ("kick") us 

drivers/vhost/vhost.c void vhost_poll_queue(struct vhost_poll *poll) · line 189

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * has made more work available on this virtqueue; Rather, we 

will continuously

drivers/vhost/vhost.c void vhost_poll_queue(struct vhost_poll *poll) · line 190

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * poll this virtqueue in the worker thread. If multiple 

virtqueues are polled,

Quoted 2 lines

drivers/vhost/vhost.c +2 -0

+ * the worker thread polls them all, e.g., in a round-robin fashion.
+ * Note that vqpoll.enabled doesn't always mean that this virtqueue 

drivers/vhost/vhost.c void vhost_poll_queue(struct vhost_poll *poll) · line 193

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * actually being polled: The backend (e.g., net.c) may 

temporarily disable it

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * using vhost_disable/enable_notify(), while vqpoll.enabled is 

drivers/vhost/vhost.c void vhost_poll_queue(struct vhost_poll *poll) · line 196

Quoted 2 lines

drivers/vhost/vhost.c +2 -0

+ *
+ * It is assumed that these functions are called relatively 

rarely, when vhost

drivers/vhost/vhost.c void vhost_poll_queue(struct vhost_poll *poll) · line 197

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * notices that this virtqueue's usage pattern significantly 

changed in a way

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * that makes polling more efficient than notification, or vice 

drivers/vhost/vhost.c void vhost_poll_queue(struct vhost_poll *poll) · line 199

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * Also, we assume that vhost_vq_disable_vqpoll() is always called on 

drivers/vhost/vhost.c void vhost_poll_queue(struct vhost_poll *poll) · line 200

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * cleanup, so any allocations done by vhost_vq_enable_vqpoll() can 

Quoted 24 lines

drivers/vhost/vhost.c +24 -0

+ * reclaimed.
+ */
+static void vhost_vq_enable_vqpoll(struct vhost_virtqueue *vq)
+{
+   if (vq->vqpoll.enabled)
+      return; /* already enabled, nothing to do */
+   if (!vq->handle_kick)
+      return; /* polling will be a waste of time if no callback! */
+   if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY)) {
+      /* vq has guest notifications enabled. Disable them,
+         and instead add vq to the polling list */
+      vhost_disable_notify(vq->dev, vq);
+      list_add_tail(&vq->vqpoll.link, &vq->dev->vqpoll_list);
+   }
+   vq->vqpoll.jiffies_last_kick = jiffies;
+   __get_user(vq->avail_idx, &vq->avail->idx);
+   vq->vqpoll.enabled = true;
+
+   /* Map userspace's vq->avail to the kernel's memory space. */
+   if (get_user_pages_fast((unsigned long)vq->avail, 1, 0,
+      &vq->vqpoll.avail_page) != 1) {
+      /* TODO: can this happen, as we check access
+      to vq->avail in advance? */
+      BUG();

drivers/vhost/vhost.c void vhost_poll_queue(struct vhost_poll *poll)

Quoted 8 lines

drivers/vhost/vhost.c +8 -0

+   }
+   vq->vqpoll.avail_mapped = (struct vring_avail *) (
+      (unsigned long)kmap(vq->vqpoll.avail_page) |
+      ((unsigned long)vq->avail & ~PAGE_MASK));
+}
+
+/*
+ * This function doesn't always succeed in changing the mode. 

drivers/vhost/vhost.c void vhost_poll_queue(struct vhost_poll *poll) · line 233

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * a temporary race condition prevents turning on guest 

Quoted 38 lines

drivers/vhost/vhost.c +32 -0

+ * vq should be polled next time again.
+ */
+static void vhost_vq_disable_vqpoll(struct vhost_virtqueue *vq)
+{
+   if (!vq->vqpoll.enabled)
+      return; /* already disabled, nothing to do */
+
+   vq->vqpoll.enabled = false;
+
+   if (!list_empty(&vq->vqpoll.link)) {
+      /* vq is on the polling list, remove it from this list and
+       * instead enable guest notifications. */
+      list_del_init(&vq->vqpoll.link);
+      if (unlikely(vhost_enable_notify(vq->dev, vq))
+         && !vq->vqpoll.shutdown) {
+         /* Race condition: guest wrote before we enabled
+          * notification, so we'll never get a notification for
+          * this work - so continue polling mode for a while. */
+         vhost_disable_notify(vq->dev, vq);
+         vq->vqpoll.enabled = true;
+         vhost_enable_notify(vq->dev, vq);
+         return;
+      }
+   }
+
+   if (vq->vqpoll.avail_mapped) {
+      kunmap(vq->vqpoll.avail_page);
+      put_page(vq->vqpoll.avail_page);
+      vq->vqpoll.avail_mapped = 0;
+   }
+}
+
 static void vhost_vq_reset(struct vhost_dev *dev,
             struct vhost_virtqueue *vq)
 {

@@ -199,6 +288,48 @@ static void vhost_vq_reset(struct vhost_dev *dev,
    vq->call = NULL;
    vq->log_ctx = NULL;

drivers/vhost/vhost.c static void vhost_vq_reset(struct vhost_dev *dev,

Quoted 8 lines

drivers/vhost/vhost.c +7 -0

    vq->memory = NULL;
+   INIT_LIST_HEAD(&vq->vqpoll.link);
+   vq->vqpoll.enabled = false;
+   vq->vqpoll.shutdown = false;
+   vq->vqpoll.avail_mapped = NULL;
+}
+
+/* roundrobin_poll() takes worker->vqpoll_list, and returns one of 

drivers/vhost/vhost.c static void vhost_vq_reset(struct vhost_dev *dev, · line 298

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * virtqueues which the caller should kick, or NULL in case none 

drivers/vhost/vhost.c static void vhost_vq_reset(struct vhost_dev *dev, · line 299

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * kicked. roundrobin_poll() also disables polling on a 

drivers/vhost/vhost.c static void vhost_vq_reset(struct vhost_dev *dev,

Quoted 3 lines

drivers/vhost/vhost.c +3 -0

+ * been polled for too long without success.
+ *
+ * This current implementation (the "round-robin" implementation) 

drivers/vhost/vhost.c static void vhost_vq_reset(struct vhost_dev *dev, · line 303

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * polls the first vq in the list, returning it or NULL as 

drivers/vhost/vhost.c static void vhost_vq_reset(struct vhost_dev *dev, · line 304

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * moves this vq to the end of the list, so next time a different one 

drivers/vhost/vhost.c static void vhost_vq_reset(struct vhost_dev *dev,

Quoted 3 lines

drivers/vhost/vhost.c +3 -0

+ * polled.
+ */
+static struct vhost_virtqueue *roundrobin_poll(struct list_head 

Quoted 100 lines

drivers/vhost/vhost.c +78 -2

+{
+   struct vhost_virtqueue *vq;
+   u16 avail_idx;
+
+   if (list_empty(list))
+      return NULL;
+
+   vq = list_first_entry(list, struct vhost_virtqueue, vqpoll.link);
+   WARN_ON(!vq->vqpoll.enabled);
+   list_move_tail(&vq->vqpoll.link, list);
+
+   /* See if there is any new work available from the guest. */
+   /* TODO: can check the optional idx feature, and if we haven't
+   * reached that idx yet, don't kick... */
+   avail_idx = vq->vqpoll.avail_mapped->idx;
+   if (avail_idx != vq->last_avail_idx)
+      return vq;
+
+   if (jiffies > vq->vqpoll.jiffies_last_kick + poll_stop_idle) {
+      /* We've been polling this virtqueue for a long time with no
+      * results, so switch back to guest notification
+      */
+      vhost_vq_disable_vqpoll(vq);
+   }
+   return NULL;
 }
 
 static int vhost_worker(void *data)

@@ -237,12 +368,62 @@ static int vhost_worker(void *data)
       spin_unlock_irq(&dev->work_lock);
 
       if (work) {
+         struct vhost_virtqueue *vq = work->vq;
          __set_current_state(TASK_RUNNING);
          work->fn(work);
+         /* Keep track of the work rate, for deciding when to
+          * enable polling */
+         if (vq) {
+            if (vq->vqpoll.jiffies_last_work != jiffies) {
+               vq->vqpoll.jiffies_last_work = jiffies;
+               vq->vqpoll.work_this_jiffy = 0;
+            }
+            vq->vqpoll.work_this_jiffy++;
+         }
+         /* If vq is in the round-robin list of virtqueues being
+          * constantly checked by this thread, move vq the end
+          * of the queue, because it had its fair chance now.
+          */
+         if (vq && !list_empty(&vq->vqpoll.link)) {
+            list_move_tail(&vq->vqpoll.link,
+               &dev->vqpoll_list);
+         }
+         /* Otherwise, if this vq is looking for notifications
+          * but vq polling is not enabled for it, do it now.
+          */
+         else if (poll_start_rate && vq && vq->handle_kick &&
+            !vq->vqpoll.enabled &&
+            !vq->vqpoll.shutdown &&
+            !(vq->used_flags & VRING_USED_F_NO_NOTIFY) &&
+            vq->vqpoll.work_this_jiffy >=
+               poll_start_rate) {
+            vhost_vq_enable_vqpoll(vq);
+         }
+      }
+      /* Check one virtqueue from the round-robin list */
+      if (!list_empty(&dev->vqpoll_list)) {
+         struct vhost_virtqueue *vq;
+
+         vq = roundrobin_poll(&dev->vqpoll_list);
+
+         if (vq) {
+            vq->handle_kick(&vq->poll.work);
+            vq->vqpoll.jiffies_last_kick = jiffies;
+         }
+
+         /* If our polling list isn't empty, ask to continue
+          * running this thread, don't yield.
+          */
+         __set_current_state(TASK_RUNNING);
          if (need_resched())
             schedule();
-      } else
-         schedule();
+      } else {
+         if (work) {
+            if (need_resched())
+               schedule();
+         } else
+            schedule();
+      }
 
    }
    unuse_mm(dev->mm);

@@ -306,6 +487,7 @@ void vhost_dev_init(struct vhost_dev *dev,
    dev->mm = NULL;
    spin_lock_init(&dev->work_lock);
    INIT_LIST_HEAD(&dev->work_list);
+   INIT_LIST_HEAD(&dev->vqpoll_list);
    dev->worker = NULL;
 

drivers/vhost/vhost.c static int vhost_attach_cgroups(struct vhost_dev

Quoted 11 lines

drivers/vhost/vhost.c +1 -1

    for (i = 0; i < dev->nvqs; ++i) {

@@ -318,7 +500,7 @@ void vhost_dev_init(struct vhost_dev *dev,
       vhost_vq_reset(dev, vq);
       if (vq->handle_kick)
          vhost_poll_init(&vq->poll, vq->handle_kick,
-               POLLIN, dev);
+               POLLIN, vq);
    }
 }
 EXPORT_SYMBOL_GPL(vhost_dev_init);

@@ -350,7 +532,7 @@ static int vhost_attach_cgroups(struct vhost_dev 

Quoted 12 lines

drivers/vhost/vhost.c +1 -1

    struct vhost_attach_cgroups_struct attach;
 
    attach.owner = current;
-   vhost_work_init(&attach.work, vhost_attach_cgroups_work);
+   vhost_work_init(&attach.work, NULL, vhost_attach_cgroups_work);
    vhost_work_queue(dev, &attach.work);
    vhost_work_flush(dev, &attach.work);
    return attach.ret;

@@ -444,6 +626,26 @@ void vhost_dev_stop(struct vhost_dev *dev)
 }
 EXPORT_SYMBOL_GPL(vhost_dev_stop);
 

drivers/vhost/vhost.c void vhost_dev_stop(struct vhost_dev *dev) · line 629

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+/* shutdown_vqpoll() asks the worker thread to shut down virtqueue 

drivers/vhost/vhost.c void vhost_dev_stop(struct vhost_dev *dev) · line 630

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * mode for a given virtqueue which is itself being shut down. We ask 

drivers/vhost/vhost.c void vhost_dev_stop(struct vhost_dev *dev) · line 631

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * worker thread to do this rather than doing it directly, so that we 

drivers/vhost/vhost.c void vhost_dev_cleanup(struct vhost_dev *dev,

Quoted 21 lines

drivers/vhost/vhost.c +17 -0

+ * race with the worker thread's use of the queue.
+ */
+static void shutdown_vqpoll_work(struct vhost_work *work)
+{
+   work->vq->vqpoll.shutdown = true;
+   vhost_vq_disable_vqpoll(work->vq);
+   WARN_ON(work->vq->vqpoll.avail_mapped);
+}
+
+static void shutdown_vqpoll(struct vhost_virtqueue *vq)
+{
+   struct vhost_work work;
+
+   vhost_work_init(&work, vq, shutdown_vqpoll_work);
+   vhost_work_queue(vq->dev, &work);
+   vhost_work_flush(vq->dev, &work);
+}
 /* Caller should have device mutex if and only if locked is set */
 void vhost_dev_cleanup(struct vhost_dev *dev, bool locked)
 {

@@ -460,6 +662,7 @@ void vhost_dev_cleanup(struct vhost_dev *dev, 

bool locked)

drivers/vhost/vhost.c bool vhost_enable_notify(struct vhost_dev

Quoted 8 lines

drivers/vhost/vhost.c +1 -0

          eventfd_ctx_put(dev->vqs[i]->call_ctx);
       if (dev->vqs[i]->call)
          fput(dev->vqs[i]->call);
+      shutdown_vqpoll(dev->vqs[i]);
       vhost_vq_reset(dev, dev->vqs[i]);
    }
    vhost_dev_free_iovecs(dev);

@@ -1491,6 +1694,19 @@ bool vhost_enable_notify(struct vhost_dev 

*dev, struct vhost_virtqueue *vq)

Quoted 5 lines

drivers/vhost/vhost.c +2 -0

    u16 avail_idx;
    int r;
 
+   /* In polling mode, when the backend (e.g., net.c) asks to enable
+    * notifications, we don't enable guest notifications. Instead, 

Quoted 8 lines

drivers/vhost/vhost.c +8 -0

+    * polling on this vq by adding it to the round-robin list.
+    */
+   if (vq->vqpoll.enabled) {
+      if (list_empty(&vq->vqpoll.link)) {
+         list_add_tail(&vq->vqpoll.link,
+            &vq->dev->vqpoll_list);
+         vq->vqpoll.jiffies_last_kick = jiffies;
+      }

drivers/vhost/vhost.c void vhost_disable_notify(struct vhost_dev

Quoted 7 lines

drivers/vhost/vhost.c +3 -0

+      return false;
+   }
+
    if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY))
       return false;
    vq->used_flags &= ~VRING_USED_F_NO_NOTIFY;

@@ -1528,6 +1744,17 @@ void vhost_disable_notify(struct vhost_dev 

*dev, struct vhost_virtqueue *vq)

Quoted 4 lines

drivers/vhost/vhost.c +1 -0

 {
    int r;
 
+   /* If this virtqueue is vqpoll.enabled, and on the polling list, 

drivers/vhost/vhost.c void vhost_disable_notify(struct vhost_dev · line 1748

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+    * will generate notifications even if the guest is asked not to 

Quoted 23 lines

drivers/vhost/vhost.c +9 -0

+    * them. So we must remove it from the round-robin polling list.
+    * Note that vqpoll.enabled remains set.
+    */
+   if (vq->vqpoll.enabled) {
+      if (!list_empty(&vq->vqpoll.link))
+         list_del_init(&vq->vqpoll.link);
+      return;
+   }
+
    if (vq->used_flags & VRING_USED_F_NO_NOTIFY)
       return;
    vq->used_flags |= VRING_USED_F_NO_NOTIFY;

drivers/vhost/vhost.h +1 -0

index 3eda654..11aaaf4 100644
@@ -24,6 +24,7 @@ struct vhost_work {
    int           flushing;
    unsigned        queue_seq;
    unsigned        done_seq;
+   struct vhost_virtqueue    *vq;
 };
 

drivers/vhost/vhost.h struct vhost_poll { · line 41

Quoted 7 lines

drivers/vhost/vhost.h +1 -1

 /* Poll a file (eventfd or socket) */

@@ -37,11 +38,12 @@ struct vhost_poll {
    struct vhost_dev    *dev;
 };
 
-void vhost_work_init(struct vhost_work *work, vhost_work_fn_t fn);
+void vhost_work_init(struct vhost_work *work, struct vhost_virtqueue 

drivers/vhost/vhost.h struct vhost_poll { · line 43

Quoted 2 lines

drivers/vhost/vhost.h +1 -0

+                     vhost_work_fn_t fn);
 void vhost_work_queue(struct vhost_dev *dev, struct vhost_work 

Quoted 53 lines

drivers/vhost/vhost.h +30 -3

 void vhost_poll_init(struct vhost_poll *poll, vhost_work_fn_t fn,
-           unsigned long mask, struct vhost_dev *dev);
+           unsigned long mask, struct vhost_virtqueue  *vq);
 int vhost_poll_start(struct vhost_poll *poll, struct file *file);
 void vhost_poll_stop(struct vhost_poll *poll);
 void vhost_poll_flush(struct vhost_poll *poll);

@@ -54,8 +56,6 @@ struct vhost_log {
    u64 len;
 };
 
-struct vhost_virtqueue;
-
 /* The virtqueue structure describes a queue attached to a device. */
 struct vhost_virtqueue {
    struct vhost_dev *dev;

@@ -110,6 +110,35 @@ struct vhost_virtqueue {
    /* Log write descriptors */
    void __user *log_base;
    struct vhost_log *log;
+   struct {
+      /* When a virtqueue is in vqpoll.enabled mode, it declares
+       * that instead of using guest notifications (kicks) to
+       * discover new work, we prefer to continuously poll this
+       * virtqueue in the worker thread.
+       * If !enabled, the rest of the fields below are undefined.
+       */
+      bool enabled;
+      /* vqpoll.enabled doesn't always mean that this virtqueue is
+       * actually being polled: The backend (e.g., net.c) may
+       * temporarily disable it using vhost_disable/enable_notify().
+       * vqpoll.link is used to maintain the thread's round-robin
+       * list of virtqueues that actually need to be polled.
+       * Note list_empty(link) means this virtqueue isn't polled.
+       */
+      struct list_head link;
+      /* If this flag is true, the virtqueue is being shut down,
+       * so vqpoll should not be re-enabled.
+       */
+      bool shutdown;
+      /* Various counters used to decide when to enter polling mode
+       * or leave it and return to notification mode.
+       */
+      unsigned long jiffies_last_kick;
+      unsigned long jiffies_last_work;
+      int work_this_jiffy;
+      struct page *avail_page;
+      volatile struct vring_avail *avail_mapped;
+   } vqpoll;
 };
 
 struct vhost_dev {

drivers/vhost/vhost.h struct vhost_dev { · line 158

Quoted 7 lines

drivers/vhost/vhost.h +1 -0

@@ -123,6 +152,7 @@ struct vhost_dev {
    spinlock_t work_lock;
    struct list_head work_list;
    struct task_struct *worker;
+   struct list_head vqpoll_list;
 };
 
 void vhost_dev_init(struct vhost_dev *, struct vhost_virtqueue 

**vqs, int nvqs);

Quoted 2 lines

Signature · 1 line

1.7.9.5

From: "Michael S. Tsirkin" <[email protected]>
To: Razya Ladelsky/Haifa/IBM@IBMIL,
Cc: [email protected], Alex Glikson/Haifa/IBM@IBMIL, Eran Raichstein/Haifa/IBM@IBMIL, Yossi Kuperman1/Haifa/IBM@IBMIL, Joel Nider/Haifa/IBM@IBMIL, [email protected], linux- [email protected], [email protected], [email protected]
Date: 10/08/2014 10:45 PM
Subject: Re: [PATCH] vhost: Add polling mode

Quoted 4 lines · Razya Ladelsky

From: Razya Ladelsky <[email protected]>
Date: Thu, 31 Jul 2014 09:47:20 +0300
Subject: [PATCH] vhost: Add polling mode

Quoted 1 line

When vhost is waiting for buffers from the guest driver (e.g.,

more packets to

Quoted 1 line

send in vhost-net's transmit queue), it normally goes to sleep and

waits for the

Quoted 1 line

guest to "kick" it. This kick involves a PIO in the guest, and

therefore an exit

Quoted 1 line

(and possibly userspace involvement in translating this PIO exit into

Quoted 3 lines

descriptor event), all of which hurts performance.

If the system is under-utilized (has cpu time to spare), vhost can

continuously

Quoted 1 line

poll the virtqueues for new buffers, and avoid asking the guest to

Quoted 1 line

This patch adds an optional polling mode to vhost, that can be enabled

Quoted 4 lines

kernel module parameter, "poll_start_rate".

When polling is active for a virtqueue, the guest is asked to disable notification (kicks), and the worker thread continuously checks

for new buffers.

Quoted 1 line

When it does discover new buffers, it simulates a "kick" by invoking

Quoted 1 line

underlying backend driver (such as vhost-net), which thinks it got

a real kick

Quoted 1 line

from the guest, and acts accordingly. If the underlying driver

asks not to be

Quoted 3 lines

kicked, we disable polling on this virtqueue.

We start polling on a virtqueue when we notice it has work to do.

Quoted 1 line

this virtqueue is later disabled after 3 seconds of polling

turning up no new

Quoted 1 line

work, as in this case we are better off returning to the exit-

based notification

Quoted 3 lines

mechanism. The default timeout of 3 seconds can be changed with the "poll_stop_idle" kernel module parameter.

Quoted 1 line

This polling approach makes lot of sense for new HW with posted-

interrupts for

Quoted 1 line

which we have exitless host-to-guest notifications. But even with

support for

Quoted 1 line

posted interrupts, guest-to-host communication still causes exits.

Polling adds

Quoted 3 lines

the missing part.

When systems are overloaded, there won't be enough cpu time for the

Quoted 1 line

vhost threads to poll their guests' devices. For these scenarios,

we plan to add

Quoted 1 line

support for vhost threads that can be shared by multiple devices, even

Quoted 2 lines

multiple vms.
Our ultimate goal is to implement the I/O acceleration features

described in:

Quoted 6 lines

KVM Forum 2013: Efficient and Scalable Virtio (by Abel Gordon)
https://www.youtube.com/watch?v=9EyweibHfEs
and
https://www.mail-archive.com/[email protected]/msg98179.html

I ran some experiments with TCP stream netperf and filebench

(having 2 threads

Quoted 4 lines

performing random reads) benchmarks on an IBM System x3650 M4. I have two machines, A and B. A hosts the vms, B runs the netserver. The vms (on A) run netperf, its destination server is running on B. All runs loaded the guests in a way that they were (cpu)

saturated. For example,

Quoted 1 line

I ran netperf with 64B messages, which is heavily loading the vm

(which is why

Quoted 2 lines

its throughput is low).
The idea was to get it 100% loaded, so we can see that the polling

is getting it

Quoted 1 line

to produce higher throughput.

And, did your tests actually produce 100% load on both host CPUs?

Quoted 1 line

The system had two cores per guest, as to allow for both the vcpu

and the vhost

Quoted 1 line

thread to run concurrently for maximum throughput (but I didn't

pin the threads

Quoted 2 lines

to specific cores).
My experiments were fair in a sense that for both cases, with or

Quoted 1 line

polling, I run both threads, vcpu and vhost, on 2 cores (set their

affinity that

Quoted 6 lines

way). The only difference was whether polling was enabled/disabled.

Results:

Netperf, 1 vm:
The polling patch improved throughput by ~33% (1516 MB/sec -> 2046

Quoted 2 lines

Number of exits/sec decreased 6x.
The same improvement was shown when I tested with 3 vms running

Quoted 4 lines

(4086 MB/sec -> 5545 MB/sec).

filebench, 1 vm:
ops/sec improved by 13% with the polling patch. Number of exits

was reduced by

Quoted 2 lines

31%.
The same experiment with 3 vms running filebench showed similar

Quoted 6 lines

Signed-off-by Razya Ladelsky <[email protected]>

---
 drivers/vhost/net.c   |    6 +-
 drivers/vhost/scsi.c  |    6 +-
 drivers/vhost/vhost.c |  245 ++++++++++++++++++++++++++++++++++++

+++++++++++--

Quoted 3 lines

 drivers/vhost/vhost.h |   38 +++++++-
 4 files changed, 277 insertions(+), 18 deletions(-)

drivers/vhost/net.c static int vhost_net_open(struct inode

Quoted 5 lines

drivers/vhost/net.c +0 -0

index 971a760..558aecb 100644
@@ -742,8 +742,10 @@ static int vhost_net_open(struct inode 

*inode, struct file *f)

Quoted 4 lines

drivers/vhost/net.c +0 -1

    }
    vhost_dev_init(dev, vqs, VHOST_NET_VQ_MAX);
 
-   vhost_poll_init(n->poll + VHOST_NET_VQ_TX, handle_tx_net, POLLOUT, 

drivers/vhost/net.c static int vhost_net_open(struct inode · line 746

Quoted 1 line

drivers/vhost/net.c +0 -1

-   vhost_poll_init(n->poll + VHOST_NET_VQ_RX, handle_rx_net, POLLIN, 

Quoted 5 lines

drivers/vhost/net.c +4 -0

+   vhost_poll_init(n->poll + VHOST_NET_VQ_TX, handle_tx_net, POLLOUT,
+         vqs[VHOST_NET_VQ_TX]);
+   vhost_poll_init(n->poll + VHOST_NET_VQ_RX, handle_rx_net, POLLIN,
+         vqs[VHOST_NET_VQ_RX]);
 

drivers/vhost/scsi.c static int vhost_scsi_open(struct inode

Quoted 7 lines

drivers/vhost/net.c +0 -0

    f->private_data = n;
 

drivers/vhost/scsi.c +0 -0

index 4f4ffa4..665eeeb 100644
@@ -1528,9 +1528,9 @@ static int vhost_scsi_open(struct inode 

*inode, struct file *f)

Quoted 4 lines

drivers/vhost/scsi.c +0 -1

    if (!vqs)
       goto err_vqs;
 
-   vhost_work_init(&vs->vs_completion_work, 

Quoted 13 lines

drivers/vhost/scsi.c +3 -2

-   vhost_work_init(&vs->vs_event_work, tcm_vhost_evt_work);
-
+   vhost_work_init(&vs->vs_completion_work, NULL,
+                  vhost_scsi_complete_cmd_work);
+   vhost_work_init(&vs->vs_event_work, NULL, tcm_vhost_evt_work);
    vs->vs_events_nr = 0;
    vs->vs_events_missed = false;
 

drivers/vhost/vhost.c +0 -0

index c90f437..fbe8174 100644

drivers/vhost/vhost.c line 33

Quoted 10 lines

drivers/vhost/vhost.c +4 -0

@@ -24,9 +24,17 @@
 #include <linux/slab.h>
 #include <linux/kthread.h>
 #include <linux/cgroup.h>
+#include <linux/jiffies.h>
 #include <linux/module.h>
 
 #include "vhost.h"
+static int poll_start_rate = 0;
+module_param(poll_start_rate, int, S_IRUGO|S_IWUSR);
+MODULE_PARM_DESC(poll_start_rate, "Start continuous polling of 

virtqueue when rate of events is at least this number per jiffy. If 0, never start polling.");

drivers/vhost/vhost.c

Quoted 4 lines

drivers/vhost/vhost.c +4 -0

+
+static int poll_stop_idle = 3*HZ; /* 3 seconds */
+module_param(poll_stop_idle, int, S_IRUGO|S_IWUSR);
+MODULE_PARM_DESC(poll_stop_idle, "Stop continuous polling of 

virtqueue after this many jiffies of no work.");

drivers/vhost/vhost.c static int vhost_poll_wakeup(wait_queue_t

Quoted 4 lines

drivers/vhost/vhost.c +0 -0

 enum {
    VHOST_MEMORY_MAX_NREGIONS = 64,

@@ -58,27 +66,28 @@ static int vhost_poll_wakeup(wait_queue_t 

*wait, unsigned mode, int sync,

drivers/vhost/vhost.c static int vhost_poll_wakeup(wait_queue_t *wait, unsigned mode, int sync,

Quoted 5 lines

drivers/vhost/vhost.c +1 -1

    return 0;
 }
 
-void vhost_work_init(struct vhost_work *work, vhost_work_fn_t fn)
+void vhost_work_init(struct vhost_work *work, struct vhost_virtqueue 

Quoted 28 lines

drivers/vhost/vhost.c +5 -4

+                     vhost_work_fn_t fn)
 {
    INIT_LIST_HEAD(&work->node);
    work->fn = fn;
    init_waitqueue_head(&work->done);
    work->flushing = 0;
    work->queue_seq = work->done_seq = 0;
+   work->vq = vq;
 }
 EXPORT_SYMBOL_GPL(vhost_work_init);
 
 /* Init poll structure */
 void vhost_poll_init(struct vhost_poll *poll, vhost_work_fn_t fn,
-           unsigned long mask, struct vhost_dev *dev)
+           unsigned long mask, struct vhost_virtqueue *vq)
 {
    init_waitqueue_func_entry(&poll->wait, vhost_poll_wakeup);
    init_poll_funcptr(&poll->table, vhost_poll_func);
    poll->mask = mask;
-   poll->dev = dev;
+   poll->dev = vq->dev;
    poll->wqh = NULL;
-
-   vhost_work_init(&poll->work, fn);
+   vhost_work_init(&poll->work, vq, fn);
 }
 EXPORT_SYMBOL_GPL(vhost_poll_init);
 

drivers/vhost/vhost.c void vhost_poll_queue(struct vhost_poll *poll) · line 186

Quoted 5 lines

drivers/vhost/vhost.c +1 -0

@@ -174,6 +183,86 @@ void vhost_poll_queue(struct vhost_poll *poll)
 }
 EXPORT_SYMBOL_GPL(vhost_poll_queue);
 
+/* Enable or disable virtqueue polling (vqpoll.enabled) for a 

drivers/vhost/vhost.c void vhost_poll_queue(struct vhost_poll *poll) · line 188

Quoted 2 lines

drivers/vhost/vhost.c +2 -0

+ *
+ * Enabling this mode it tells the guest not to notify ("kick") us 

drivers/vhost/vhost.c void vhost_poll_queue(struct vhost_poll *poll) · line 189

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * has made more work available on this virtqueue; Rather, we 

will continuously

drivers/vhost/vhost.c void vhost_poll_queue(struct vhost_poll *poll) · line 190

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * poll this virtqueue in the worker thread. If multiple 

virtqueues are polled,

Quoted 2 lines

drivers/vhost/vhost.c +2 -0

+ * the worker thread polls them all, e.g., in a round-robin fashion.
+ * Note that vqpoll.enabled doesn't always mean that this virtqueue 

drivers/vhost/vhost.c void vhost_poll_queue(struct vhost_poll *poll) · line 193

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * actually being polled: The backend (e.g., net.c) may 

temporarily disable it

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * using vhost_disable/enable_notify(), while vqpoll.enabled is 

drivers/vhost/vhost.c void vhost_poll_queue(struct vhost_poll *poll) · line 196

Quoted 2 lines

drivers/vhost/vhost.c +2 -0

+ *
+ * It is assumed that these functions are called relatively 

rarely, when vhost

drivers/vhost/vhost.c void vhost_poll_queue(struct vhost_poll *poll) · line 197

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * notices that this virtqueue's usage pattern significantly 

changed in a way

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * that makes polling more efficient than notification, or vice 

drivers/vhost/vhost.c void vhost_poll_queue(struct vhost_poll *poll) · line 199

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * Also, we assume that vhost_vq_disable_vqpoll() is always called on 

drivers/vhost/vhost.c void vhost_poll_queue(struct vhost_poll *poll) · line 200

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * cleanup, so any allocations done by vhost_vq_enable_vqpoll() can 

Quoted 24 lines

drivers/vhost/vhost.c +24 -0

+ * reclaimed.
+ */
+static void vhost_vq_enable_vqpoll(struct vhost_virtqueue *vq)
+{
+   if (vq->vqpoll.enabled)
+      return; /* already enabled, nothing to do */
+   if (!vq->handle_kick)
+      return; /* polling will be a waste of time if no callback! */
+   if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY)) {
+      /* vq has guest notifications enabled. Disable them,
+         and instead add vq to the polling list */
+      vhost_disable_notify(vq->dev, vq);
+      list_add_tail(&vq->vqpoll.link, &vq->dev->vqpoll_list);
+   }
+   vq->vqpoll.jiffies_last_kick = jiffies;
+   __get_user(vq->avail_idx, &vq->avail->idx);
+   vq->vqpoll.enabled = true;
+
+   /* Map userspace's vq->avail to the kernel's memory space. */
+   if (get_user_pages_fast((unsigned long)vq->avail, 1, 0,
+      &vq->vqpoll.avail_page) != 1) {
+      /* TODO: can this happen, as we check access
+      to vq->avail in advance? */
+      BUG();

drivers/vhost/vhost.c void vhost_poll_queue(struct vhost_poll *poll)

Quoted 8 lines

drivers/vhost/vhost.c +8 -0

+   }
+   vq->vqpoll.avail_mapped = (struct vring_avail *) (
+      (unsigned long)kmap(vq->vqpoll.avail_page) |
+      ((unsigned long)vq->avail & ~PAGE_MASK));
+}
+
+/*
+ * This function doesn't always succeed in changing the mode. 

drivers/vhost/vhost.c void vhost_poll_queue(struct vhost_poll *poll) · line 233

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * a temporary race condition prevents turning on guest 

Quoted 38 lines

drivers/vhost/vhost.c +32 -0

+ * vq should be polled next time again.
+ */
+static void vhost_vq_disable_vqpoll(struct vhost_virtqueue *vq)
+{
+   if (!vq->vqpoll.enabled)
+      return; /* already disabled, nothing to do */
+
+   vq->vqpoll.enabled = false;
+
+   if (!list_empty(&vq->vqpoll.link)) {
+      /* vq is on the polling list, remove it from this list and
+       * instead enable guest notifications. */
+      list_del_init(&vq->vqpoll.link);
+      if (unlikely(vhost_enable_notify(vq->dev, vq))
+         && !vq->vqpoll.shutdown) {
+         /* Race condition: guest wrote before we enabled
+          * notification, so we'll never get a notification for
+          * this work - so continue polling mode for a while. */
+         vhost_disable_notify(vq->dev, vq);
+         vq->vqpoll.enabled = true;
+         vhost_enable_notify(vq->dev, vq);
+         return;
+      }
+   }
+
+   if (vq->vqpoll.avail_mapped) {
+      kunmap(vq->vqpoll.avail_page);
+      put_page(vq->vqpoll.avail_page);
+      vq->vqpoll.avail_mapped = 0;
+   }
+}
+
 static void vhost_vq_reset(struct vhost_dev *dev,
             struct vhost_virtqueue *vq)
 {

@@ -199,6 +288,48 @@ static void vhost_vq_reset(struct vhost_dev *dev,
    vq->call = NULL;
    vq->log_ctx = NULL;

drivers/vhost/vhost.c static void vhost_vq_reset(struct vhost_dev *dev,

Quoted 8 lines

drivers/vhost/vhost.c +7 -0

    vq->memory = NULL;
+   INIT_LIST_HEAD(&vq->vqpoll.link);
+   vq->vqpoll.enabled = false;
+   vq->vqpoll.shutdown = false;
+   vq->vqpoll.avail_mapped = NULL;
+}
+
+/* roundrobin_poll() takes worker->vqpoll_list, and returns one of 

drivers/vhost/vhost.c static void vhost_vq_reset(struct vhost_dev *dev, · line 298

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * virtqueues which the caller should kick, or NULL in case none 

drivers/vhost/vhost.c static void vhost_vq_reset(struct vhost_dev *dev, · line 299

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * kicked. roundrobin_poll() also disables polling on a 

drivers/vhost/vhost.c static void vhost_vq_reset(struct vhost_dev *dev,

Quoted 3 lines

drivers/vhost/vhost.c +3 -0

+ * been polled for too long without success.
+ *
+ * This current implementation (the "round-robin" implementation) 

drivers/vhost/vhost.c static void vhost_vq_reset(struct vhost_dev *dev, · line 303

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * polls the first vq in the list, returning it or NULL as 

drivers/vhost/vhost.c static void vhost_vq_reset(struct vhost_dev *dev, · line 304

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * moves this vq to the end of the list, so next time a different one 

drivers/vhost/vhost.c static void vhost_vq_reset(struct vhost_dev *dev,

Quoted 3 lines

drivers/vhost/vhost.c +3 -0

+ * polled.
+ */
+static struct vhost_virtqueue *roundrobin_poll(struct list_head 

Quoted 100 lines

drivers/vhost/vhost.c +78 -2

+{
+   struct vhost_virtqueue *vq;
+   u16 avail_idx;
+
+   if (list_empty(list))
+      return NULL;
+
+   vq = list_first_entry(list, struct vhost_virtqueue, vqpoll.link);
+   WARN_ON(!vq->vqpoll.enabled);
+   list_move_tail(&vq->vqpoll.link, list);
+
+   /* See if there is any new work available from the guest. */
+   /* TODO: can check the optional idx feature, and if we haven't
+   * reached that idx yet, don't kick... */
+   avail_idx = vq->vqpoll.avail_mapped->idx;
+   if (avail_idx != vq->last_avail_idx)
+      return vq;
+
+   if (jiffies > vq->vqpoll.jiffies_last_kick + poll_stop_idle) {
+      /* We've been polling this virtqueue for a long time with no
+      * results, so switch back to guest notification
+      */
+      vhost_vq_disable_vqpoll(vq);
+   }
+   return NULL;
 }
 
 static int vhost_worker(void *data)

@@ -237,12 +368,62 @@ static int vhost_worker(void *data)
       spin_unlock_irq(&dev->work_lock);
 
       if (work) {
+         struct vhost_virtqueue *vq = work->vq;
          __set_current_state(TASK_RUNNING);
          work->fn(work);
+         /* Keep track of the work rate, for deciding when to
+          * enable polling */
+         if (vq) {
+            if (vq->vqpoll.jiffies_last_work != jiffies) {
+               vq->vqpoll.jiffies_last_work = jiffies;
+               vq->vqpoll.work_this_jiffy = 0;
+            }
+            vq->vqpoll.work_this_jiffy++;
+         }
+         /* If vq is in the round-robin list of virtqueues being
+          * constantly checked by this thread, move vq the end
+          * of the queue, because it had its fair chance now.
+          */
+         if (vq && !list_empty(&vq->vqpoll.link)) {
+            list_move_tail(&vq->vqpoll.link,
+               &dev->vqpoll_list);
+         }
+         /* Otherwise, if this vq is looking for notifications
+          * but vq polling is not enabled for it, do it now.
+          */
+         else if (poll_start_rate && vq && vq->handle_kick &&
+            !vq->vqpoll.enabled &&
+            !vq->vqpoll.shutdown &&
+            !(vq->used_flags & VRING_USED_F_NO_NOTIFY) &&
+            vq->vqpoll.work_this_jiffy >=
+               poll_start_rate) {
+            vhost_vq_enable_vqpoll(vq);
+         }
+      }
+      /* Check one virtqueue from the round-robin list */
+      if (!list_empty(&dev->vqpoll_list)) {
+         struct vhost_virtqueue *vq;
+
+         vq = roundrobin_poll(&dev->vqpoll_list);
+
+         if (vq) {
+            vq->handle_kick(&vq->poll.work);
+            vq->vqpoll.jiffies_last_kick = jiffies;
+         }
+
+         /* If our polling list isn't empty, ask to continue
+          * running this thread, don't yield.
+          */
+         __set_current_state(TASK_RUNNING);
          if (need_resched())
             schedule();
-      } else
-         schedule();
+      } else {
+         if (work) {
+            if (need_resched())
+               schedule();
+         } else
+            schedule();
+      }
 
    }
    unuse_mm(dev->mm);

@@ -306,6 +487,7 @@ void vhost_dev_init(struct vhost_dev *dev,
    dev->mm = NULL;
    spin_lock_init(&dev->work_lock);
    INIT_LIST_HEAD(&dev->work_list);
+   INIT_LIST_HEAD(&dev->vqpoll_list);
    dev->worker = NULL;
 

drivers/vhost/vhost.c static int vhost_attach_cgroups(struct vhost_dev

Quoted 11 lines

drivers/vhost/vhost.c +1 -1

    for (i = 0; i < dev->nvqs; ++i) {

@@ -318,7 +500,7 @@ void vhost_dev_init(struct vhost_dev *dev,
       vhost_vq_reset(dev, vq);
       if (vq->handle_kick)
          vhost_poll_init(&vq->poll, vq->handle_kick,
-               POLLIN, dev);
+               POLLIN, vq);
    }
 }
 EXPORT_SYMBOL_GPL(vhost_dev_init);

@@ -350,7 +532,7 @@ static int vhost_attach_cgroups(struct vhost_dev 

Quoted 12 lines

drivers/vhost/vhost.c +1 -1

    struct vhost_attach_cgroups_struct attach;
 
    attach.owner = current;
-   vhost_work_init(&attach.work, vhost_attach_cgroups_work);
+   vhost_work_init(&attach.work, NULL, vhost_attach_cgroups_work);
    vhost_work_queue(dev, &attach.work);
    vhost_work_flush(dev, &attach.work);
    return attach.ret;

@@ -444,6 +626,26 @@ void vhost_dev_stop(struct vhost_dev *dev)
 }
 EXPORT_SYMBOL_GPL(vhost_dev_stop);
 

drivers/vhost/vhost.c void vhost_dev_stop(struct vhost_dev *dev) · line 629

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+/* shutdown_vqpoll() asks the worker thread to shut down virtqueue 

drivers/vhost/vhost.c void vhost_dev_stop(struct vhost_dev *dev) · line 630

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * mode for a given virtqueue which is itself being shut down. We ask 

drivers/vhost/vhost.c void vhost_dev_stop(struct vhost_dev *dev) · line 631

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+ * worker thread to do this rather than doing it directly, so that we 

drivers/vhost/vhost.c void vhost_dev_cleanup(struct vhost_dev *dev,

Quoted 21 lines

drivers/vhost/vhost.c +17 -0

+ * race with the worker thread's use of the queue.
+ */
+static void shutdown_vqpoll_work(struct vhost_work *work)
+{
+   work->vq->vqpoll.shutdown = true;
+   vhost_vq_disable_vqpoll(work->vq);
+   WARN_ON(work->vq->vqpoll.avail_mapped);
+}
+
+static void shutdown_vqpoll(struct vhost_virtqueue *vq)
+{
+   struct vhost_work work;
+
+   vhost_work_init(&work, vq, shutdown_vqpoll_work);
+   vhost_work_queue(vq->dev, &work);
+   vhost_work_flush(vq->dev, &work);
+}
 /* Caller should have device mutex if and only if locked is set */
 void vhost_dev_cleanup(struct vhost_dev *dev, bool locked)
 {

@@ -460,6 +662,7 @@ void vhost_dev_cleanup(struct vhost_dev *dev, 

bool locked)

drivers/vhost/vhost.c bool vhost_enable_notify(struct vhost_dev

Quoted 8 lines

drivers/vhost/vhost.c +1 -0

          eventfd_ctx_put(dev->vqs[i]->call_ctx);
       if (dev->vqs[i]->call)
          fput(dev->vqs[i]->call);
+      shutdown_vqpoll(dev->vqs[i]);
       vhost_vq_reset(dev, dev->vqs[i]);
    }
    vhost_dev_free_iovecs(dev);

@@ -1491,6 +1694,19 @@ bool vhost_enable_notify(struct vhost_dev 

*dev, struct vhost_virtqueue *vq)

Quoted 5 lines

drivers/vhost/vhost.c +2 -0

    u16 avail_idx;
    int r;
 
+   /* In polling mode, when the backend (e.g., net.c) asks to enable
+    * notifications, we don't enable guest notifications. Instead, 

Quoted 8 lines

drivers/vhost/vhost.c +8 -0

+    * polling on this vq by adding it to the round-robin list.
+    */
+   if (vq->vqpoll.enabled) {
+      if (list_empty(&vq->vqpoll.link)) {
+         list_add_tail(&vq->vqpoll.link,
+            &vq->dev->vqpoll_list);
+         vq->vqpoll.jiffies_last_kick = jiffies;
+      }

drivers/vhost/vhost.c void vhost_disable_notify(struct vhost_dev

Quoted 7 lines

drivers/vhost/vhost.c +3 -0

+      return false;
+   }
+
    if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY))
       return false;
    vq->used_flags &= ~VRING_USED_F_NO_NOTIFY;

@@ -1528,6 +1744,17 @@ void vhost_disable_notify(struct vhost_dev 

*dev, struct vhost_virtqueue *vq)

Quoted 4 lines

drivers/vhost/vhost.c +1 -0

 {
    int r;
 
+   /* If this virtqueue is vqpoll.enabled, and on the polling list, 

drivers/vhost/vhost.c void vhost_disable_notify(struct vhost_dev · line 1748

Quoted 1 line

drivers/vhost/vhost.c +1 -0

+    * will generate notifications even if the guest is asked not to 

Quoted 23 lines

drivers/vhost/vhost.c +9 -0

+    * them. So we must remove it from the round-robin polling list.
+    * Note that vqpoll.enabled remains set.
+    */
+   if (vq->vqpoll.enabled) {
+      if (!list_empty(&vq->vqpoll.link))
+         list_del_init(&vq->vqpoll.link);
+      return;
+   }
+
    if (vq->used_flags & VRING_USED_F_NO_NOTIFY)
       return;
    vq->used_flags |= VRING_USED_F_NO_NOTIFY;

drivers/vhost/vhost.h +1 -0

index 3eda654..11aaaf4 100644
@@ -24,6 +24,7 @@ struct vhost_work {
    int           flushing;
    unsigned        queue_seq;
    unsigned        done_seq;
+   struct vhost_virtqueue    *vq;
 };
 

drivers/vhost/vhost.h struct vhost_poll { · line 41

Quoted 7 lines

drivers/vhost/vhost.h +1 -1

 /* Poll a file (eventfd or socket) */

@@ -37,11 +38,12 @@ struct vhost_poll {
    struct vhost_dev    *dev;
 };
 
-void vhost_work_init(struct vhost_work *work, vhost_work_fn_t fn);
+void vhost_work_init(struct vhost_work *work, struct vhost_virtqueue 

drivers/vhost/vhost.h struct vhost_poll { · line 43

Quoted 2 lines

drivers/vhost/vhost.h +1 -0

+                     vhost_work_fn_t fn);
 void vhost_work_queue(struct vhost_dev *dev, struct vhost_work 

Quoted 53 lines

drivers/vhost/vhost.h +30 -3

 void vhost_poll_init(struct vhost_poll *poll, vhost_work_fn_t fn,
-           unsigned long mask, struct vhost_dev *dev);
+           unsigned long mask, struct vhost_virtqueue  *vq);
 int vhost_poll_start(struct vhost_poll *poll, struct file *file);
 void vhost_poll_stop(struct vhost_poll *poll);
 void vhost_poll_flush(struct vhost_poll *poll);

@@ -54,8 +56,6 @@ struct vhost_log {
    u64 len;
 };
 
-struct vhost_virtqueue;
-
 /* The virtqueue structure describes a queue attached to a device. */
 struct vhost_virtqueue {
    struct vhost_dev *dev;

@@ -110,6 +110,35 @@ struct vhost_virtqueue {
    /* Log write descriptors */
    void __user *log_base;
    struct vhost_log *log;
+   struct {
+      /* When a virtqueue is in vqpoll.enabled mode, it declares
+       * that instead of using guest notifications (kicks) to
+       * discover new work, we prefer to continuously poll this
+       * virtqueue in the worker thread.
+       * If !enabled, the rest of the fields below are undefined.
+       */
+      bool enabled;
+      /* vqpoll.enabled doesn't always mean that this virtqueue is
+       * actually being polled: The backend (e.g., net.c) may
+       * temporarily disable it using vhost_disable/enable_notify().
+       * vqpoll.link is used to maintain the thread's round-robin
+       * list of virtqueues that actually need to be polled.
+       * Note list_empty(link) means this virtqueue isn't polled.
+       */
+      struct list_head link;
+      /* If this flag is true, the virtqueue is being shut down,
+       * so vqpoll should not be re-enabled.
+       */
+      bool shutdown;
+      /* Various counters used to decide when to enter polling mode
+       * or leave it and return to notification mode.
+       */
+      unsigned long jiffies_last_kick;
+      unsigned long jiffies_last_work;
+      int work_this_jiffy;
+      struct page *avail_page;
+      volatile struct vring_avail *avail_mapped;
+   } vqpoll;
 };
 
 struct vhost_dev {

drivers/vhost/vhost.h struct vhost_dev { · line 158

Quoted 7 lines

drivers/vhost/vhost.h +1 -0

@@ -123,6 +152,7 @@ struct vhost_dev {
    spinlock_t work_lock;
    struct list_head work_list;
    struct task_struct *worker;
+   struct list_head vqpoll_list;
 };
 
 void vhost_dev_init(struct vhost_dev *, struct vhost_virtqueue 

**vqs, int nvqs);

Quoted 2 lines

Signature · 1 line

1.7.9.5

From: Razya Ladelsky <[email protected]>
Date: Thu, 31 Jul 2014 09:47:20 +0300
Subject: [PATCH] vhost: Add polling mode

When vhost is waiting for buffers from the guest driver (e.g., more packets to send in vhost-net's transmit queue), it normally goes to sleep and waits for the guest to "kick" it. This kick involves a PIO in the guest, and therefore an exit (and possibly userspace involvement in translating this PIO exit into a file descriptor event), all of which hurts performance.

If the system is under-utilized (has cpu time to spare), vhost can continuously poll the virtqueues for new buffers, and avoid asking the guest to kick us. This patch adds an optional polling mode to vhost, that can be enabled via a kernel module parameter, "poll_start_rate".

When polling is active for a virtqueue, the guest is asked to disable notification (kicks), and the worker thread continuously checks for new buffers. When it does discover new buffers, it simulates a "kick" by invoking the underlying backend driver (such as vhost-net), which thinks it got a real kick from the guest, and acts accordingly. If the underlying driver asks not to be kicked, we disable polling on this virtqueue.

We start polling on a virtqueue when we notice it has work to do. Polling on this virtqueue is later disabled after 3 seconds of polling turning up no new work, as in this case we are better off returning to the exit-based notification mechanism. The default timeout of 3 seconds can be changed with the "poll_stop_idle" kernel module parameter.

This polling approach makes lot of sense for new HW with posted-interrupts for which we have exitless host-to-guest notifications. But even with support for posted interrupts, guest-to-host communication still causes exits. Polling adds the missing part.

When systems are overloaded, there won't be enough cpu time for the various vhost threads to poll their guests' devices. For these scenarios, we plan to add support for vhost threads that can be shared by multiple devices, even of multiple vms.
Our ultimate goal is to implement the I/O acceleration features described in:
KVM Forum 2013: Efficient and Scalable Virtio (by Abel Gordon)
https://www.youtube.com/watch?v=9EyweibHfEs
and
https://www.mail-archive.com/[email protected]/msg98179.html

I ran some experiments with TCP stream netperf and filebench (having 2 threads performing random reads) benchmarks on an IBM System x3650 M4.
I have two machines, A and B. A hosts the vms, B runs the netserver. The vms (on A) run netperf, its destination server is running on B. All runs loaded the guests in a way that they were (cpu) saturated. For example, I ran netperf with 64B messages, which is heavily loading the vm (which is why its throughput is low).
The idea was to get it 100% loaded, so we can see that the polling is getting it to produce higher throughput.

The system had two cores per guest, as to allow for both the vcpu and the vhost thread to run concurrently for maximum throughput (but I didn't pin the threads to specific cores).
My experiments were fair in a sense that for both cases, with or without polling, I run both threads, vcpu and vhost, on 2 cores (set their affinity that way). The only difference was whether polling was enabled/disabled.

Results:

Netperf, 1 vm:
The polling patch improved throughput by ~33% (1516 MB/sec -> 2046 MB/sec). Number of exits/sec decreased 6x.
The same improvement was shown when I tested with 3 vms running netperf (4086 MB/sec -> 5545 MB/sec).

filebench, 1 vm:
ops/sec improved by 13% with the polling patch. Number of exits was reduced by 31%.
The same experiment with 3 vms running filebench showed similar numbers.

Signed-off-by Razya Ladelsky <[email protected]>

Quoted 68 lines · Razya Ladelsky

From: Razya Ladelsky <[email protected]>
Date: Thu, 31 Jul 2014 09:47:20 +0300
Subject: [PATCH] vhost: Add polling mode

When vhost is waiting for buffers from the guest driver (e.g., more packets to send in vhost-net's transmit queue), it normally goes to sleep and waits for the guest to "kick" it. This kick involves a PIO in the guest, and therefore an exit (and possibly userspace involvement in translating this PIO exit into a file descriptor event), all of which hurts performance.

If the system is under-utilized (has cpu time to spare), vhost can continuously poll the virtqueues for new buffers, and avoid asking the guest to kick us. This patch adds an optional polling mode to vhost, that can be enabled via a kernel module parameter, "poll_start_rate".

When polling is active for a virtqueue, the guest is asked to disable notification (kicks), and the worker thread continuously checks for new buffers. When it does discover new buffers, it simulates a "kick" by invoking the underlying backend driver (such as vhost-net), which thinks it got a real kick from the guest, and acts accordingly. If the underlying driver asks not to be kicked, we disable polling on this virtqueue.

We start polling on a virtqueue when we notice it has work to do. Polling on this virtqueue is later disabled after 3 seconds of polling turning up no new work, as in this case we are better off returning to the exit-based notification mechanism. The default timeout of 3 seconds can be changed with the "poll_stop_idle" kernel module parameter.

This polling approach makes lot of sense for new HW with posted-interrupts for which we have exitless host-to-guest notifications. But even with support for posted interrupts, guest-to-host communication still causes exits. Polling adds the missing part.

When systems are overloaded, there won't be enough cpu time for the various vhost threads to poll their guests' devices. For these scenarios, we plan to add support for vhost threads that can be shared by multiple devices, even of multiple vms.
Our ultimate goal is to implement the I/O acceleration features described in:
KVM Forum 2013: Efficient and Scalable Virtio (by Abel Gordon)
https://www.youtube.com/watch?v=9EyweibHfEs
and
https://www.mail-archive.com/[email protected]/msg98179.html

I ran some experiments with TCP stream netperf and filebench (having 2 threads performing random reads) benchmarks on an IBM System x3650 M4.
I have two machines, A and B. A hosts the vms, B runs the netserver. The vms (on A) run netperf, its destination server is running on B. All runs loaded the guests in a way that they were (cpu) saturated. For example, I ran netperf with 64B messages, which is heavily loading the vm (which is why its throughput is low).
The idea was to get it 100% loaded, so we can see that the polling is getting it to produce higher throughput.

The system had two cores per guest, as to allow for both the vcpu and the vhost thread to run concurrently for maximum throughput (but I didn't pin the threads to specific cores).
My experiments were fair in a sense that for both cases, with or without polling, I run both threads, vcpu and vhost, on 2 cores (set their affinity that way). The only difference was whether polling was enabled/disabled.

Results:

Netperf, 1 vm:
The polling patch improved throughput by ~33% (1516 MB/sec -> 2046 MB/sec). Number of exits/sec decreased 6x.
The same improvement was shown when I tested with 3 vms running netperf (4086 MB/sec -> 5545 MB/sec).

Quoted 6 lines

filebench, 1 vm:
ops/sec improved by 13% with the polling patch. Number of exits was reduced by 31%.
The same experiment with 3 vms running filebench showed similar numbers.

Signed-off-by Razya Ladelsky <[email protected]>

Gave it a quick try on s390/kvm. As expected it makes no difference for big streaming workload like iperf. uperf with a 1-1 round robin got indeed faster by about 30%.
The high CPU consumption is something that bothers me though, as virtualized systems tend to be full.

drivers/vhost/vhost.c line 37

Quoted 7 lines

drivers/vhost/vhost.c +7 -0

+static int poll_start_rate = 0;
+module_param(poll_start_rate, int, S_IRUGO|S_IWUSR);
+MODULE_PARM_DESC(poll_start_rate, "Start continuous polling of virtqueue when rate of events is at least this number per jiffy. If 0, never start polling.");
+
+static int poll_stop_idle = 3*HZ; /* 3 seconds */
+module_param(poll_stop_idle, int, S_IRUGO|S_IWUSR);
+MODULE_PARM_DESC(poll_stop_idle, "Stop continuous polling of virtqueue after this many jiffies of no work.");

This seems ridicoudly high. Even one jiffie is an eternity, so setting it to 1 as a default would reduce the CPU overhead for most cases. If we dont have a packet in one millisecond, we can surely go back to the kick approach, I think.

Christian

Quoted 5 lines

Results:

Netperf, 1 vm:
The polling patch improved throughput by ~33% (1516 MB/sec -> 2046

Quoted 2 lines

Number of exits/sec decreased 6x.
The same improvement was shown when I tested with 3 vms running

Quoted 4 lines

(4086 MB/sec -> 5545 MB/sec).

filebench, 1 vm:
ops/sec improved by 13% with the polling patch. Number of exits

was reduced by

Quoted 2 lines

31%.
The same experiment with 3 vms running filebench showed similar

Quoted 2 lines

Signed-off-by Razya Ladelsky <[email protected]>

Gave it a quick try on s390/kvm. As expected it makes no difference for big streaming workload like iperf.
uperf with a 1-1 round robin got indeed faster by about 30%. The high CPU consumption is something that bothers me though, as virtualized systems tend to be full.

drivers/vhost/vhost.c

Quoted 3 lines

drivers/vhost/vhost.c +3 -0

+static int poll_start_rate = 0;
+module_param(poll_start_rate, int, S_IRUGO|S_IWUSR);
+MODULE_PARM_DESC(poll_start_rate, "Start continuous polling of 

virtqueue when rate of events is at least this number per jiffy. If 0, never start polling.");

drivers/vhost/vhost.c

Quoted 4 lines

drivers/vhost/vhost.c +4 -0

+
+static int poll_stop_idle = 3*HZ; /* 3 seconds */
+module_param(poll_stop_idle, int, S_IRUGO|S_IWUSR);
+MODULE_PARM_DESC(poll_stop_idle, "Stop continuous polling of 

virtqueue after this many jiffies of no work.");

This seems ridicoudly high. Even one jiffie is an eternity, so setting it to 1 as a default would reduce the CPU overhead for most

If we dont have a packet in one millisecond, we can surely go back to the kick approach, I think.

Christian

Quoted 15 lines

Quoted 11 lines

Results:

Netperf, 1 vm:
The polling patch improved throughput by ~33% (1516 MB/sec -> 2046 MB/sec). Number of exits/sec decreased 6x.
The same improvement was shown when I tested with 3 vms running netperf (4086 MB/sec -> 5545 MB/sec).

filebench, 1 vm:
ops/sec improved by 13% with the polling patch. Number of exits

was reduced by

Quoted 3 lines

31%.
The same experiment with 3 vms running filebench showed similar numbers.

Quoted 9 lines

Quoted 1 line

Signed-off-by Razya Ladelsky <[email protected]>

Gave it a quick try on s390/kvm. As expected it makes no difference for big streaming workload like iperf.
uperf with a 1-1 round robin got indeed faster by about 30%. The high CPU consumption is something that bothers me though, as virtualized systems tend to be full.

Thanks for confirming the results!
The best way to use this patch would be along with a shared vhost thread for multiple
devices/vms, as described in:
http://domino.research.ibm.com/library/cyberdig.nsf/1e4115aea78b6e7c85256b360066f0d4/479e3578ed05bfac85257b4200427735!OpenDocument
This work assumes having a dedicated I/O core where the vhost thread serves multiple vms, which
makes the high cpu utilization less of a concern.

Quoted 13 lines

drivers/vhost/vhost.c

Quoted 5 lines

drivers/vhost/vhost.c +3 -0

+static int poll_start_rate = 0;
+module_param(poll_start_rate, int, S_IRUGO|S_IWUSR);
+MODULE_PARM_DESC(poll_start_rate, "Start continuous polling of 

virtqueue when rate of events is at least this number per jiffy. If 0, never start polling.");

drivers/vhost/vhost.c

Quoted 4 lines

drivers/vhost/vhost.c +4 -0

+
+static int poll_stop_idle = 3*HZ; /* 3 seconds */
+module_param(poll_stop_idle, int, S_IRUGO|S_IWUSR);
+MODULE_PARM_DESC(poll_stop_idle, "Stop continuous polling of 

virtqueue after this many jiffies of no work.");

Quoted 7 lines

This seems ridicoudly high. Even one jiffie is an eternity, so setting it to 1 as a default would reduce the CPU overhead for most cases. If we dont have a packet in one millisecond, we can surely go back to the kick approach, I think.

Christian

Good point, will reduce it and recheck.
Thank you,
Razya

Quoted 25 lines

Quoted 15 lines

Quoted 11 lines

Results:

Netperf, 1 vm:
The polling patch improved throughput by ~33% (1516 MB/sec -> 2046 MB/sec). Number of exits/sec decreased 6x.
The same improvement was shown when I tested with 3 vms running netperf (4086 MB/sec -> 5545 MB/sec).

filebench, 1 vm:
ops/sec improved by 13% with the polling patch. Number of exits

was reduced by

Quoted 3 lines

31%.
The same experiment with 3 vms running filebench showed similar numbers.

Quoted 9 lines

Quoted 1 line

Signed-off-by Razya Ladelsky <[email protected]>

Gave it a quick try on s390/kvm. As expected it makes no difference for big streaming workload like iperf.
uperf with a 1-1 round robin got indeed faster by about 30%. The high CPU consumption is something that bothers me though, as virtualized systems tend to be full.

Quoted 9 lines

Thanks for confirming the results!
The best way to use this patch would be along with a shared vhost thread for multiple
devices/vms, as described in:
http://domino.research.ibm.com/library/cyberdig.nsf/1e4115aea78b6e7c85256b360066f0d4/479e3578ed05bfac85257b4200427735!OpenDocument
This work assumes having a dedicated I/O core where the vhost thread serves multiple vms, which
makes the high cpu utilization less of a concern.

Hi, Razya, Shirley
I am going to test the combination of
"several (depends on total number of cpu on host, e.g., total_number * 1/3) vhost threads server all VMs" and "vhost: add polling mode", now I get the patch "http://thread.gmane.org/gmane.comp.emulators.kvm.devel/88682/focus=88723" posted by Shirley, any update to this patch?

And, I want to make a bit change on this patch, create total_cpu_number * 1/N(N={3,4}) vhost threads instead of per-cpu vhost thread to server all VMs,

any ideas?

Thanks,
Zhang Haoyu

Quoted 24 lines

Quoted 13 lines

drivers/vhost/vhost.c

Quoted 5 lines

drivers/vhost/vhost.c +3 -0

+static int poll_start_rate = 0;
+module_param(poll_start_rate, int, S_IRUGO|S_IWUSR);
+MODULE_PARM_DESC(poll_start_rate, "Start continuous polling of 

virtqueue when rate of events is at least this number per jiffy. If 0, never start polling.");

drivers/vhost/vhost.c

Quoted 4 lines

drivers/vhost/vhost.c +4 -0

+
+static int poll_stop_idle = 3*HZ; /* 3 seconds */
+module_param(poll_stop_idle, int, S_IRUGO|S_IWUSR);
+MODULE_PARM_DESC(poll_stop_idle, "Stop continuous polling of 

virtqueue after this many jiffies of no work.");

Quoted 7 lines

This seems ridicoudly high. Even one jiffie is an eternity, so setting it to 1 as a default would reduce the CPU overhead for most cases. If we dont have a packet in one millisecond, we can surely go back to the kick approach, I think.

Christian

Good point, will reduce it and recheck.
Thank you,
Razya

From: Razya Ladelsky <[email protected]>
Date: Thu, 31 Jul 2014 09:47:20 +0300
Subject: [PATCH] vhost: Add polling mode

When vhost is waiting for buffers from the guest driver (e.g., more packets to send in vhost-net's transmit queue), it normally goes to sleep and waits for the guest to "kick" it. This kick involves a PIO in the guest, and therefore an exit (and possibly userspace involvement in translating this PIO exit into a file descriptor event), all of which hurts performance.

If the system is under-utilized (has cpu time to spare), vhost can continuously poll the virtqueues for new buffers, and avoid asking the guest to kick us. This patch adds an optional polling mode to vhost, that can be enabled via a kernel module parameter, "poll_start_rate".

When polling is active for a virtqueue, the guest is asked to disable notification (kicks), and the worker thread continuously checks for new buffers. When it does discover new buffers, it simulates a "kick" by invoking the underlying backend driver (such as vhost-net), which thinks it got a real kick from the guest, and acts accordingly. If the underlying driver asks not to be kicked, we disable polling on this virtqueue.

We start polling on a virtqueue when we notice it has work to do. Polling on this virtqueue is later disabled after 3 seconds of polling turning up no new work, as in this case we are better off returning to the exit-based notification mechanism. The default timeout of 3 seconds can be changed with the "poll_stop_idle" kernel module parameter.

This polling approach makes lot of sense for new HW with posted-interrupts for which we have exitless host-to-guest notifications. But even with support for posted interrupts, guest-to-host communication still causes exits. Polling adds the missing part.

When systems are overloaded, there won't be enough cpu time for the various vhost threads to poll their guests' devices. For these scenarios, we plan to add support for vhost threads that can be shared by multiple devices, even of multiple vms.
Our ultimate goal is to implement the I/O acceleration features described in:
KVM Forum 2013: Efficient and Scalable Virtio (by Abel Gordon)
https://www.youtube.com/watch?v=9EyweibHfEs
and
https://www.mail-archive.com/[email protected]/msg98179.html

I ran some experiments with TCP stream netperf and filebench (having 2 threads performing random reads) benchmarks on an IBM System x3650 M4.
I have two machines, A and B. A hosts the vms, B runs the netserver. The vms (on A) run netperf, its destination server is running on B. All runs loaded the guests in a way that they were (cpu) saturated. For example, I ran netperf with 64B messages, which is heavily loading the vm (which is why its throughput is low).
The idea was to get it 100% loaded, so we can see that the polling is getting it to produce higher throughput.

The system had two cores per guest, as to allow for both the vcpu and the vhost thread to run concurrently for maximum throughput (but I didn't pin the threads to specific cores).
My experiments were fair in a sense that for both cases, with or without polling, I run both threads, vcpu and vhost, on 2 cores (set their affinity that way). The only difference was whether polling was enabled/disabled.

Results:

Netperf, 1 vm:
The polling patch improved throughput by ~33% (1516 MB/sec -> 2046 MB/sec). Number of exits/sec decreased 6x.
The same improvement was shown when I tested with 3 vms running netperf (4086 MB/sec -> 5545 MB/sec).

filebench, 1 vm:
ops/sec improved by 13% with the polling patch. Number of exits was reduced by 31%.
The same experiment with 3 vms running filebench showed similar numbers.

Signed-off-by Razya Ladelsky <[email protected]>

---
 drivers/vhost/net.c   |    6 +-
 drivers/vhost/scsi.c  |    6 +-
 drivers/vhost/vhost.c |  245 +++++++++++++++++++++++++++++++++++++++++++++++--
 drivers/vhost/vhost.h |   38 +++++++-
 4 files changed, 277 insertions(+), 18 deletions(-)

drivers/vhost/net.c +4 -2

index 971a760..558aecb 100644
@@ -742,8 +742,10 @@ static int vhost_net_open(struct inode *inode, struct file *f)
 	}
 	vhost_dev_init(dev, vqs, VHOST_NET_VQ_MAX);
 
-	vhost_poll_init(n->poll + VHOST_NET_VQ_TX, handle_tx_net, POLLOUT, dev);
-	vhost_poll_init(n->poll + VHOST_NET_VQ_RX, handle_rx_net, POLLIN, dev);
+	vhost_poll_init(n->poll + VHOST_NET_VQ_TX, handle_tx_net, POLLOUT,
+			vqs[VHOST_NET_VQ_TX]);
+	vhost_poll_init(n->poll + VHOST_NET_VQ_RX, handle_rx_net, POLLIN,
+			vqs[VHOST_NET_VQ_RX]);
 
 	f->private_data = n;
 

drivers/vhost/scsi.c +3 -3

index 4f4ffa4..665eeeb 100644
@@ -1528,9 +1528,9 @@ static int vhost_scsi_open(struct inode *inode, struct file *f)
 	if (!vqs)
 		goto err_vqs;
 
-	vhost_work_init(&vs->vs_completion_work, vhost_scsi_complete_cmd_work);
-	vhost_work_init(&vs->vs_event_work, tcm_vhost_evt_work);
-
+	vhost_work_init(&vs->vs_completion_work, NULL,
+						vhost_scsi_complete_cmd_work);
+	vhost_work_init(&vs->vs_event_work, NULL, tcm_vhost_evt_work);
 	vs->vs_events_nr = 0;
 	vs->vs_events_missed = false;
 

drivers/vhost/vhost.c +1 -0

index c90f437..fbe8174 100644
@@ -24,9 +24,17 @@
 #include <linux/slab.h>
 #include <linux/kthread.h>
 #include <linux/cgroup.h>
+#include <linux/jiffies.h>
 #include <linux/module.h>
 
 #include "vhost.h"

drivers/vhost/vhost.c +22 -5

@@ -58,27 +66,28 @@ static int vhost_poll_wakeup(wait_queue_t *wait, unsigned mode, int sync,
 	return 0;
 }
 
-void vhost_work_init(struct vhost_work *work, vhost_work_fn_t fn)
+void vhost_work_init(struct vhost_work *work, struct vhost_virtqueue *vq,
+							vhost_work_fn_t fn)
 {
 	INIT_LIST_HEAD(&work->node);
 	work->fn = fn;
 	init_waitqueue_head(&work->done);
 	work->flushing = 0;
 	work->queue_seq = work->done_seq = 0;
+	work->vq = vq;
 }
 EXPORT_SYMBOL_GPL(vhost_work_init);
 
 /* Init poll structure */
 void vhost_poll_init(struct vhost_poll *poll, vhost_work_fn_t fn,
-		     unsigned long mask, struct vhost_dev *dev)
+		     unsigned long mask, struct vhost_virtqueue *vq)
 {
 	init_waitqueue_func_entry(&poll->wait, vhost_poll_wakeup);
 	init_poll_funcptr(&poll->table, vhost_poll_func);
 	poll->mask = mask;
-	poll->dev = dev;
+	poll->dev = vq->dev;
 	poll->wqh = NULL;
-
-	vhost_work_init(&poll->work, fn);
+	vhost_work_init(&poll->work, vq, fn);
 }
 EXPORT_SYMBOL_GPL(vhost_poll_init);
 

@@ -174,6 +183,86 @@ void vhost_poll_queue(struct vhost_poll *poll)
 }
 EXPORT_SYMBOL_GPL(vhost_poll_queue);
 
+/* Enable or disable virtqueue polling (vqpoll.enabled) for a virtqueue.
+ *
+ * Enabling this mode it tells the guest not to notify ("kick") us when its
+ * has made more work available on this virtqueue; Rather, we will continuously
+ * poll this virtqueue in the worker thread. If multiple virtqueues are polled,
+ * the worker thread polls them all, e.g., in a round-robin fashion.
+ * Note that vqpoll.enabled doesn't always mean that this virtqueue is
+ * actually being polled: The backend (e.g., net.c) may temporarily disable it
+ * using vhost_disable/enable_notify(), while vqpoll.enabled is unchanged.
+ *
+ * It is assumed that these functions are called relatively rarely, when vhost
+ * notices that this virtqueue's usage pattern significantly changed in a way
+ * that makes polling more efficient than notification, or vice versa.
+ * Also, we assume that vhost_vq_disable_vqpoll() is always called on vq
+ * cleanup, so any allocations done by vhost_vq_enable_vqpoll() can be
+ * reclaimed.

drivers/vhost/vhost.c +118 -0

+		BUG();
+	}
+	vq->vqpoll.avail_mapped = (struct vring_avail *) (
+		(unsigned long)kmap(vq->vqpoll.avail_page) |
+		((unsigned long)vq->avail & ~PAGE_MASK));
+}
+
+/*
+ * This function doesn't always succeed in changing the mode. Sometimes
+ * a temporary race condition prevents turning on guest notifications, so
+ * vq should be polled next time again.
+ */
+static void vhost_vq_disable_vqpoll(struct vhost_virtqueue *vq)
+{
+	if (!vq->vqpoll.enabled)
+		return; /* already disabled, nothing to do */
+
+	vq->vqpoll.enabled = false;
+
+	if (!list_empty(&vq->vqpoll.link)) {
+		/* vq is on the polling list, remove it from this list and
+		 * instead enable guest notifications. */
+		list_del_init(&vq->vqpoll.link);
+		if (unlikely(vhost_enable_notify(vq->dev, vq))
+			&& !vq->vqpoll.shutdown) {
+			/* Race condition: guest wrote before we enabled
+			 * notification, so we'll never get a notification for
+			 * this work - so continue polling mode for a while. */
+			vhost_disable_notify(vq->dev, vq);
+			vq->vqpoll.enabled = true;
+			vhost_enable_notify(vq->dev, vq);
+			return;
+		}
+	}
+
+	if (vq->vqpoll.avail_mapped) {
+		kunmap(vq->vqpoll.avail_page);
+		put_page(vq->vqpoll.avail_page);
+		vq->vqpoll.avail_mapped = 0;
+	}
+}
+
 static void vhost_vq_reset(struct vhost_dev *dev,
 			   struct vhost_virtqueue *vq)
 {

@@ -199,6 +288,48 @@ static void vhost_vq_reset(struct vhost_dev *dev,
 	vq->call = NULL;
 	vq->log_ctx = NULL;
 	vq->memory = NULL;
+	INIT_LIST_HEAD(&vq->vqpoll.link);
+	vq->vqpoll.enabled = false;
+	vq->vqpoll.shutdown = false;
+	vq->vqpoll.avail_mapped = NULL;
+}
+
+/* roundrobin_poll() takes worker->vqpoll_list, and returns one of the
+ * virtqueues which the caller should kick, or NULL in case none should be
+ * kicked. roundrobin_poll() also disables polling on a virtqueue which has
+ * been polled for too long without success.
+ *
+ * This current implementation (the "round-robin" implementation) only
+ * polls the first vq in the list, returning it or NULL as appropriate, and
+ * moves this vq to the end of the list, so next time a different one is
+ * polled.
+ */
+static struct vhost_virtqueue *roundrobin_poll(struct list_head *list)
+{
+	struct vhost_virtqueue *vq;
+	u16 avail_idx;
+
+	if (list_empty(list))
+		return NULL;
+
+	vq = list_first_entry(list, struct vhost_virtqueue, vqpoll.link);
+	WARN_ON(!vq->vqpoll.enabled);
+	list_move_tail(&vq->vqpoll.link, list);
+
+	/* See if there is any new work available from the guest. */
+	/* TODO: can check the optional idx feature, and if we haven't
+	* reached that idx yet, don't kick... */
+	avail_idx = vq->vqpoll.avail_mapped->idx;
+	if (avail_idx != vq->last_avail_idx)
+		return vq;
+
+	if (jiffies > vq->vqpoll.jiffies_last_kick + poll_stop_idle) {
+		/* We've been polling this virtqueue for a long time with no
+		* results, so switch back to guest notification
+		*/
+		vhost_vq_disable_vqpoll(vq);
+	}
+	return NULL;
 }
 
 static int vhost_worker(void *data)

@@ -237,12 +368,62 @@ static int vhost_worker(void *data)
 		spin_unlock_irq(&dev->work_lock);
 
 		if (work) {
+			struct vhost_virtqueue *vq = work->vq;
 			__set_current_state(TASK_RUNNING);
 			work->fn(work);
+			/* Keep track of the work rate, for deciding when to
+			 * enable polling */
+			if (vq) {
+				if (vq->vqpoll.jiffies_last_work != jiffies) {
+					vq->vqpoll.jiffies_last_work = jiffies;
+					vq->vqpoll.work_this_jiffy = 0;
+				}
+				vq->vqpoll.work_this_jiffy++;
+			}
+			/* If vq is in the round-robin list of virtqueues being
+			 * constantly checked by this thread, move vq the end
+			 * of the queue, because it had its fair chance now.
+			 */
+			if (vq && !list_empty(&vq->vqpoll.link)) {
+				list_move_tail(&vq->vqpoll.link,
+					&dev->vqpoll_list);
+			}
+			/* Otherwise, if this vq is looking for notifications
+			 * but vq polling is not enabled for it, do it now.
+			 */
+			else if (poll_start_rate && vq && vq->handle_kick &&
+				!vq->vqpoll.enabled &&
+				!vq->vqpoll.shutdown &&
+				!(vq->used_flags & VRING_USED_F_NO_NOTIFY) &&
+				vq->vqpoll.work_this_jiffy >=
+					poll_start_rate) {
+				vhost_vq_enable_vqpoll(vq);
+			}
+		}
+		/* Check one virtqueue from the round-robin list */
+		if (!list_empty(&dev->vqpoll_list)) {
+			struct vhost_virtqueue *vq;
+

drivers/vhost/vhost.c +8 -2

+			__set_current_state(TASK_RUNNING);
 			if (need_resched())
 				schedule();
 
-		} else
-			schedule();
+		} else {
+			if (work) {
+				if (need_resched())
+					schedule();
+			} else
+				schedule();
+		}
 
 	}

drivers/vhost/vhost.c +48 -2

 	unuse_mm(dev->mm);

@@ -306,6 +487,7 @@ void vhost_dev_init(struct vhost_dev *dev,
 	dev->mm = NULL;
 	spin_lock_init(&dev->work_lock);
 	INIT_LIST_HEAD(&dev->work_list);
+	INIT_LIST_HEAD(&dev->vqpoll_list);
 	dev->worker = NULL;
 
 	for (i = 0; i < dev->nvqs; ++i) {

@@ -318,7 +500,7 @@ void vhost_dev_init(struct vhost_dev *dev,
 		vhost_vq_reset(dev, vq);
 		if (vq->handle_kick)
 			vhost_poll_init(&vq->poll, vq->handle_kick,
-					POLLIN, dev);
+					POLLIN, vq);
 	}
 }
 EXPORT_SYMBOL_GPL(vhost_dev_init);

@@ -350,7 +532,7 @@ static int vhost_attach_cgroups(struct vhost_dev *dev)
 	struct vhost_attach_cgroups_struct attach;
 
 	attach.owner = current;
-	vhost_work_init(&attach.work, vhost_attach_cgroups_work);
+	vhost_work_init(&attach.work, NULL, vhost_attach_cgroups_work);
 	vhost_work_queue(dev, &attach.work);
 	vhost_work_flush(dev, &attach.work);
 	return attach.ret;

@@ -444,6 +626,26 @@ void vhost_dev_stop(struct vhost_dev *dev)
 }
 EXPORT_SYMBOL_GPL(vhost_dev_stop);
 
+/* shutdown_vqpoll() asks the worker thread to shut down virtqueue polling
+ * mode for a given virtqueue which is itself being shut down. We ask the
+ * worker thread to do this rather than doing it directly, so that we don't
+ * race with the worker thread's use of the queue.
+ */
+static void shutdown_vqpoll_work(struct vhost_work *work)
+{
+	work->vq->vqpoll.shutdown = true;
+	vhost_vq_disable_vqpoll(work->vq);
+	WARN_ON(work->vq->vqpoll.avail_mapped);
+}
+
+static void shutdown_vqpoll(struct vhost_virtqueue *vq)
+{
+	struct vhost_work work;
+
+	vhost_work_init(&work, vq, shutdown_vqpoll_work);
+	vhost_work_queue(vq->dev, &work);
+	vhost_work_flush(vq->dev, &work);
+}
 /* Caller should have device mutex if and only if locked is set */
 void vhost_dev_cleanup(struct vhost_dev *dev, bool locked)
 {

@@ -460,6 +662,7 @@ void vhost_dev_cleanup(struct vhost_dev *dev, bool locked)
 			eventfd_ctx_put(dev->vqs[i]->call_ctx);
 		if (dev->vqs[i]->call)
 			fput(dev->vqs[i]->call);
+		shutdown_vqpoll(dev->vqs[i]);
 		vhost_vq_reset(dev, dev->vqs[i]);
 	}
 	vhost_dev_free_iovecs(dev);

@@ -1491,6 +1694,19 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
 	u16 avail_idx;
 	int r;
 
+	/* In polling mode, when the backend (e.g., net.c) asks to enable
+	 * notifications, we don't enable guest notifications. Instead, start
+	 * polling on this vq by adding it to the round-robin list.
+	 */
+	if (vq->vqpoll.enabled) {
+		if (list_empty(&vq->vqpoll.link)) {
+			list_add_tail(&vq->vqpoll.link,
+				&vq->dev->vqpoll_list);
+			vq->vqpoll.jiffies_last_kick = jiffies;
+		}
+		return false;
+	}
+
 	if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY))
 		return false;
 	vq->used_flags &= ~VRING_USED_F_NO_NOTIFY;

@@ -1528,6 +1744,17 @@ void vhost_disable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
 {
 	int r;
 
+	/* If this virtqueue is vqpoll.enabled, and on the polling list, it
+	 * will generate notifications even if the guest is asked not to send
+	 * them. So we must remove it from the round-robin polling list.
+	 * Note that vqpoll.enabled remains set.
+	 */
+	if (vq->vqpoll.enabled) {
+		if (!list_empty(&vq->vqpoll.link))
+			list_del_init(&vq->vqpoll.link);
+		return;
+	}
+
 	if (vq->used_flags & VRING_USED_F_NO_NOTIFY)
 		return;
 	vq->used_flags |= VRING_USED_F_NO_NOTIFY;

drivers/vhost/vhost.h +17 -4

index 3eda654..11aaaf4 100644
@@ -24,6 +24,7 @@ struct vhost_work {
 	int			  flushing;
 	unsigned		  queue_seq;
 	unsigned		  done_seq;
+	struct vhost_virtqueue    *vq;
 };
 
 /* Poll a file (eventfd or socket) */

@@ -37,11 +38,12 @@ struct vhost_poll {
 	struct vhost_dev	 *dev;
 };
 
-void vhost_work_init(struct vhost_work *work, vhost_work_fn_t fn);
+void vhost_work_init(struct vhost_work *work, struct vhost_virtqueue *vq,
+							vhost_work_fn_t fn);
 void vhost_work_queue(struct vhost_dev *dev, struct vhost_work *work);
 
 void vhost_poll_init(struct vhost_poll *poll, vhost_work_fn_t fn,
-		     unsigned long mask, struct vhost_dev *dev);
+		     unsigned long mask, struct vhost_virtqueue  *vq);
 int vhost_poll_start(struct vhost_poll *poll, struct file *file);
 void vhost_poll_stop(struct vhost_poll *poll);
 void vhost_poll_flush(struct vhost_poll *poll);

@@ -54,8 +56,6 @@ struct vhost_log {
 	u64 len;
 };
 
-struct vhost_virtqueue;
-
 /* The virtqueue structure describes a queue attached to a device. */
 struct vhost_virtqueue {
 	struct vhost_dev *dev;

@@ -110,6 +110,35 @@ struct vhost_virtqueue {
 	/* Log write descriptors */
 	void __user *log_base;
 	struct vhost_log *log;
+	struct {
+      /* When a virtqueue is in vqpoll.enabled mode, it declares
+       * that instead of using guest notifications (kicks) to
+       * discover new work, we prefer to continuously poll this
+       * virtqueue in the worker thread.
+       * If !enabled, the rest of the fields below are undefined.
+       */
+		bool enabled;
+      /* vqpoll.enabled doesn't always mean that this virtqueue is
+       * actually being polled: The backend (e.g., net.c) may
+       * temporarily disable it using vhost_disable/enable_notify().
+       * vqpoll.link is used to maintain the thread's round-robin
+       * list of virtqueues that actually need to be polled.

drivers/vhost/vhost.h +7 -0

+		unsigned long jiffies_last_kick;
+		unsigned long jiffies_last_work;
+		int work_this_jiffy;
+		struct page *avail_page;
+		volatile struct vring_avail *avail_mapped;
+	} vqpoll;
 };
 
 struct vhost_dev {

@@ -123,6 +152,7 @@ struct vhost_dev {
 	spinlock_t work_lock;
 	struct list_head work_list;
 	struct task_struct *worker;
+	struct list_head vqpoll_list;
 };
 
 void vhost_dev_init(struct vhost_dev *, struct vhost_virtqueue **vqs, int nvqs);

Signature · 1 line

1.7.9.5

Quoted 4 lines

Results:

Netperf, 1 vm:
The polling patch improved throughput by ~33% (1516 MB/sec -> 2046

Quoted 2 lines

Number of exits/sec decreased 6x.
The same improvement was shown when I tested with 3 vms running

Quoted 4 lines

(4086 MB/sec -> 5545 MB/sec).

filebench, 1 vm:
ops/sec improved by 13% with the polling patch. Number of exits

was reduced by

Quoted 2 lines

31%.
The same experiment with 3 vms running filebench showed similar

Quoted 2 lines

Signed-off-by Razya Ladelsky <[email protected]>

This really needs more thourough benchmarking report, including system data. One good example for a related patch:
http://lwn.net/Articles/551179/
though for virtualization, we need data about host as well, and if you want to look at streaming benchmarks, you need to test different message sizes and measure packet size.

"Michael S. Tsirkin" <[email protected]> wrote on 20/08/2014 01:57:10 PM:

Quoted 13 lines

Quoted 13 lines

Results:

Netperf, 1 vm:
The polling patch improved throughput by ~33% (1516 MB/sec -> 2046 MB/sec). Number of exits/sec decreased 6x.
The same improvement was shown when I tested with 3 vms running netperf (4086 MB/sec -> 5545 MB/sec).

filebench, 1 vm:
ops/sec improved by 13% with the polling patch. Number of exits was reduced by 31%.
The same experiment with 3 vms running filebench showed similar numbers.

Quoted 9 lines

Quoted 1 line

Signed-off-by Razya Ladelsky <[email protected]>

This really needs more thourough benchmarking report, including system data. One good example for a related patch:
http://lwn.net/Articles/551179/
though for virtualization, we need data about host as well, and if you want to look at streaming benchmarks, you need to test different message sizes and measure packet size.

Hi Michael,
I have already tried running netperf with several message sizes:
64,128,256,512,600,800...
But the results are inconsistent even in the baseline/unpatched configuration.
For smaller msg sizes, I get consistent numbers. However, at some point, when I increase the msg size
I get unstable results. For example, for a 512B msg, I get two scenarios:
vm utilization 100%, vhost utilization 75%, throughput ~6300 vm utilization 80%, vhost utilization 13%, throughput ~9400 (line rate)