Revert "virtio-blk: support completion batching for the IRQ path"]

----- Forwarded message from "Michael S. Tsirkin" <[email protected]> -----

From: "Michael S. Tsirkin" <[email protected]>
Date: Fri, 9 Jun 2023 03:27:28 -0400
To: [email protected]
Cc: kernel test robot <[email protected]>, Suwan Kim <[email protected]>, "Roberts, Martin" <[email protected]>, Jason Wang
<[email protected]>, Paolo Bonzini <[email protected]>, Stefan Hajnoczi <[email protected]>, Xuan Zhuo <[email protected]>, Jens Axboe <[email protected]>, [email protected], [email protected]
Subject: [PATCH v2] Revert "virtio-blk: support completion batching for the IRQ path"
Message-ID: <336455b4f630f329380a8f53ee8cad3868764d5c.1686295549.git.mst@redhat.com>

This reverts commit 07b679f70d73483930e8d3c293942416d9cd5c13.

This change appears to have broken things... We now see applications hanging during disk accesses. e.g.
multi-port virtio-blk device running in h/w (FPGA) Host running a simple 'fio' test.
[global]
thread=1
direct=1
ioengine=libaio
norandommap=1
group_reporting=1
bs=4K
rw=read
iodepth=128
runtime=1
numjobs=4
time_based
[job0]
filename=/dev/vda
[job1]
filename=/dev/vdb
[job2]
filename=/dev/vdc
...
[job15]
filename=/dev/vdp

i.e. 16 disks; 4 queues per disk; simple burst of 4KB reads This is repeatedly run in a loop.

After a few, normally <10 seconds, fio hangs.
With 64 queues (16 disks), failure occurs within a few seconds; with 8 queues (2 disks) it may take ~hour before hanging. Last message:
fio-3.19
Starting 8 threads
Jobs: 1 (f=1): [_(7),R(1)][68.3%][eta 03h:11m:06s]
I think this means at the end of the run 1 queue was left incomplete.

'diskstats' (run while fio is hung) shows no outstanding transactions. e.g.

$ cat /proc/diskstats
...
252       0 vda 1843140071 0 14745120568 712568645 0 0 0 0 0 3117947 712568645 0 0 0 0 0 0
252      16 vdb 1816291511 0 14530332088 704905623 0 0 0 0 0 3117711 704905623 0 0 0 0 0 0
...

Other stats (in the h/w, and added to the virtio-blk driver ([a]virtio_queue_rq(), [b]virtblk_handle_req(), [c]virtblk_request_done()) all agree, and show every request had a completion, and that virtblk_request_done() never gets called. e.g.
PF= 0 vq=0 1 2 3

[a]request_count     -   839416590   813148916   105586179    84988123
[b]completion1_count -   839416590   813148916   105586179    84988123

[c]completion2_count - 0 0 0 0

PF= 1 vq=0 1 2 3

[a]request_count     -   823335887   812516140   104582672    75856549
[b]completion1_count -   823335887   812516140   104582672    75856549

[c]completion2_count - 0 0 0 0

i.e. the issue is after the virtio-blk driver.

This change was introduced in kernel 6.3.0.
I am seeing this using 6.3.3.
If I run with an earlier kernel (5.15), it does not occur.
If I make a simple patch to the 6.3.3 virtio-blk driver, to skip the blk_mq_add_to_batch()call, it does not fail. e.g.
kernel 5.15 - this is OK
virtio_blk.c,virtblk_done() [irq handler]

                 if (likely(!blk_should_fake_timeout(req->q))) {
                          blk_mq_complete_request(req);
                 }

kernel 6.3.3 - this fails
virtio_blk.c,virtblk_handle_req() [irq handler]

                 if (likely(!blk_should_fake_timeout(req->q))) {
                          if (!blk_mq_complete_request_remote(req)) {
                                  if (!blk_mq_add_to_batch(req, iob, virtblk_vbr_status(vbr), virtblk_complete_batch)) {
                                           virtblk_request_done(req);    //this never gets called... so blk_mq_add_to_batch() must always succeed
                                   }
                          }
                 }

If I do, kernel 6.3.3 - this is OK
virtio_blk.c,virtblk_handle_req() [irq handler]

                 if (likely(!blk_should_fake_timeout(req->q))) {
                          if (!blk_mq_complete_request_remote(req)) {
                                   virtblk_request_done(req); //force this here...
                                  if (!blk_mq_add_to_batch(req, iob, virtblk_vbr_status(vbr), virtblk_complete_batch)) {
                                           virtblk_request_done(req);    //this never gets called... so blk_mq_add_to_batch() must always succeed
                                   }
                          }
                 }

Perhaps you might like to fix/test/revert this change... Martin

---

Since v1:
fix build error

Still completely untested as I'm traveling. Martin, Suwan, could you please test and report? Suwan if you have a better revert in mind pls post and I will be happy to drop this.

Thanks!

 drivers/block/virtio_blk.c | 82 +++++++++++++++++---------------------
 1 file changed, 37 insertions(+), 45 deletions(-)

index 2b918e28acaa..b47358da92a2 100644
@@ -348,63 +348,33 @@ static inline void virtblk_request_done(struct request *req)
 	blk_mq_end_request(req, status);
 }
 
-static void virtblk_complete_batch(struct io_comp_batch *iob)
-{
-	struct request *req;
-
-	rq_list_for_each(&iob->req_list, req) {
-		virtblk_unmap_data(req, blk_mq_rq_to_pdu(req));
-		virtblk_cleanup_cmd(req);
-	}
-	blk_mq_end_request_batch(iob);
-}
-
-static int virtblk_handle_req(struct virtio_blk_vq *vq,
-			      struct io_comp_batch *iob)
-{
-	struct virtblk_req *vbr;
-	int req_done = 0;
-	unsigned int len;
-
-	while ((vbr = virtqueue_get_buf(vq->vq, &len)) != NULL) {
-		struct request *req = blk_mq_rq_from_pdu(vbr);
-
-		if (likely(!blk_should_fake_timeout(req->q)) &&
-		    !blk_mq_complete_request_remote(req) &&
-		    !blk_mq_add_to_batch(req, iob, virtblk_vbr_status(vbr),
-					 virtblk_complete_batch))
-			virtblk_request_done(req);
-		req_done++;
-	}
-
-	return req_done;
-}
-
 static void virtblk_done(struct virtqueue *vq)
 {
 	struct virtio_blk *vblk = vq->vdev->priv;
-	struct virtio_blk_vq *vblk_vq = &vblk->vqs[vq->index];
-	int req_done = 0;
+	bool req_done = false;
+	int qid = vq->index;
+	struct virtblk_req *vbr;
 	unsigned long flags;
-	DEFINE_IO_COMP_BATCH(iob);
+	unsigned int len;
 
-	spin_lock_irqsave(&vblk_vq->lock, flags);
+	spin_lock_irqsave(&vblk->vqs[qid].lock, flags);
 	do {
 		virtqueue_disable_cb(vq);
-		req_done += virtblk_handle_req(vblk_vq, &iob);
+		while ((vbr = virtqueue_get_buf(vblk->vqs[qid].vq, &len)) != NULL) {
+			struct request *req = blk_mq_rq_from_pdu(vbr);
 
+			if (likely(!blk_should_fake_timeout(req->q)))
+				blk_mq_complete_request(req);
+			req_done = true;
+		}
 		if (unlikely(virtqueue_is_broken(vq)))
 			break;
 	} while (!virtqueue_enable_cb(vq));
 
-	if (req_done) {
-		if (!rq_list_empty(iob.req_list))
-			iob.complete(&iob);
-
-		/* In case queue is stopped waiting for more buffers. */
+	/* In case queue is stopped waiting for more buffers. */
+	if (req_done)
 		blk_mq_start_stopped_hw_queues(vblk->disk->queue, true);
-	}
-	spin_unlock_irqrestore(&vblk_vq->lock, flags);
+	spin_unlock_irqrestore(&vblk->vqs[qid].lock, flags);
 }
 
 static void virtio_commit_rqs(struct blk_mq_hw_ctx *hctx)

@@ -1283,15 +1253,37 @@ static void virtblk_map_queues(struct blk_mq_tag_set *set)
 	}
 }
 
+static void virtblk_complete_batch(struct io_comp_batch *iob)
+{
+	struct request *req;
+
+	rq_list_for_each(&iob->req_list, req) {
+		virtblk_unmap_data(req, blk_mq_rq_to_pdu(req));
+		virtblk_cleanup_cmd(req);
+	}
+	blk_mq_end_request_batch(iob);
+}
+
 static int virtblk_poll(struct blk_mq_hw_ctx *hctx, struct io_comp_batch *iob)
 {
 	struct virtio_blk *vblk = hctx->queue->queuedata;
 	struct virtio_blk_vq *vq = get_virtio_blk_vq(hctx);
+	struct virtblk_req *vbr;
 	unsigned long flags;
+	unsigned int len;
 	int found = 0;
 
 	spin_lock_irqsave(&vq->lock, flags);
-	found = virtblk_handle_req(vq, iob);
+
+	while ((vbr = virtqueue_get_buf(vq->vq, &len)) != NULL) {
+		struct request *req = blk_mq_rq_from_pdu(vbr);
+
+		found++;
+		if (!blk_mq_complete_request_remote(req) &&
+		    !blk_mq_add_to_batch(req, iob, virtblk_vbr_status(vbr),
+						virtblk_complete_batch))
+			virtblk_request_done(req);
+	}
 
 	if (found)
 		blk_mq_start_stopped_hw_queues(vblk->disk->queue, true);

MST

----- End forwarded message -----

What commit id is this in Linus's tree?

thanks,

greg k-h

afd384f0dbea2229fd11159efb86a5b41051c4a9

MST

Great, and what tree(s) do you want it applied to, just 6.3.y?

thanks,

greg k-h

That's the only one that has the commit we revert so yes. Thanks!

Great, now queued up, thanks.

greg k-h