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use simple waitqueue for kvm vcpu waitqueue (v2)

PATCH In review Marcelo Tosatti 5 participants 14 messages Nov 25, 2014 – Dec 5, 2014

Subsystems (10)
Arm (32-bit) x86 KVM PowerPC +6 more subsystems
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Marcelo Tosatti

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#0 lore raw

cover use simple waitqueue for kvm vcpu waitqueue (v2)

The problem:

On -RT, an emulated LAPIC timer instances has the following path:

1) hard interrupt
2) ksoftirqd is scheduled
3) ksoftirqd wakes up vcpu thread
4) vcpu thread is scheduled

This extra context switch introduces unnecessary latency in the LAPIC path for a KVM guest.

The solution:

Allow waking up vcpu thread from hardirq context, thus avoiding the need for ksoftirqd to be scheduled.

Normal waitqueues make use of spinlocks, which on -RT are sleepable locks. Therefore, waking up a waitqueue waiter involves locking a sleeping lock, which is not allowed from hard interrupt context.

cyclictest command line:

# cyclictest -m -n -q -p99 -l 1000000 -h60  -D 1m

This patch reduces the average latency in my tests from 14us to 11us.

v2: improve changelog (Rik van Riel)

author
#1 lore raw

PATCH 1/2 KVM: use simple waitqueue for vcpu->wq

The problem:

On -RT, an emulated LAPIC timer instances has the following path:

1) hard interrupt
2) ksoftirqd is scheduled
3) ksoftirqd wakes up vcpu thread
4) vcpu thread is scheduled

This extra context switch introduces unnecessary latency in the LAPIC path for a KVM guest.

The solution:

Allow waking up vcpu thread from hardirq context, thus avoiding the need for ksoftirqd to be scheduled.

Normal waitqueues make use of spinlocks, which on -RT are sleepable locks. Therefore, waking up a waitqueue waiter involves locking a sleeping lock, which is not allowed from hard interrupt context.

cyclictest command line:

# cyclictest -m -n -q -p99 -l 1000000 -h60  -D 1m

This patch reduces the average latency in my tests from 14us to 11us.

Signed-off-by Marcelo Tosatti <[email protected]>
---
 arch/arm/kvm/arm.c                  |    4 ++--
 arch/arm/kvm/psci.c                 |    4 ++--
 arch/mips/kvm/kvm_mips.c            |    8 ++++----
 arch/powerpc/include/asm/kvm_host.h |    4 ++--
 arch/powerpc/kvm/book3s_hv.c        |   20 ++++++++++----------
 arch/s390/include/asm/kvm_host.h    |    2 +-
 arch/s390/kvm/interrupt.c           |   22 ++++++++++------------
 arch/s390/kvm/sigp.c                |   16 ++++++++--------
 arch/x86/kvm/lapic.c                |    6 +++---
 include/linux/kvm_host.h            |    4 ++--
 virt/kvm/async_pf.c                 |    4 ++--
 virt/kvm/kvm_main.c                 |   16 ++++++++--------
 12 files changed, 54 insertions(+), 56 deletions(-)
arch/arm/kvm/arm.c +2 -2
Index: linux-stable-rt/arch/arm/kvm/arm.c
===================================================================
@@ -495,9 +495,9 @@
 
 static void vcpu_pause(struct kvm_vcpu *vcpu)
 {
-	wait_queue_head_t *wq = kvm_arch_vcpu_wq(vcpu);
+	struct swait_head *wq = kvm_arch_vcpu_wq(vcpu);
 
-	wait_event_interruptible(*wq, !vcpu->arch.pause);
+	swait_event_interruptible(*wq, !vcpu->arch.pause);
 }
 
 static int kvm_vcpu_initialized(struct kvm_vcpu *vcpu)
arch/arm/kvm/psci.c +2 -2
Index: linux-stable-rt/arch/arm/kvm/psci.c
===================================================================
@@ -36,7 +36,7 @@
 {
 	struct kvm *kvm = source_vcpu->kvm;
 	struct kvm_vcpu *vcpu = NULL, *tmp;
-	wait_queue_head_t *wq;
+	struct swait_head *wq;
 	unsigned long cpu_id;
 	unsigned long mpidr;
 	phys_addr_t target_pc;

@@ -80,7 +80,7 @@
 	smp_mb();		/* Make sure the above is visible */
 
 	wq = kvm_arch_vcpu_wq(vcpu);
-	wake_up_interruptible(wq);
+	swait_wake_interruptible(wq);
 
 	return KVM_PSCI_RET_SUCCESS;
 }
arch/mips/kvm/kvm_mips.c +4 -4
Index: linux-stable-rt/arch/mips/kvm/kvm_mips.c
===================================================================
@@ -464,8 +464,8 @@
 
 	dvcpu->arch.wait = 0;
 
-	if (waitqueue_active(&dvcpu->wq)) {
-		wake_up_interruptible(&dvcpu->wq);
+	if (swaitqueue_active(&dvcpu->wq)) {
+		swait_wake_interruptible(&dvcpu->wq);
 	}
 
 	return 0;

@@ -971,8 +971,8 @@
 	kvm_mips_callbacks->queue_timer_int(vcpu);
 
 	vcpu->arch.wait = 0;
-	if (waitqueue_active(&vcpu->wq)) {
-		wake_up_interruptible(&vcpu->wq);
+	if (swaitqueue_active(&vcpu->wq)) {
+		swait_wake_nterruptible(&vcpu->wq);
 	}
 }
arch/powerpc/include/asm/kvm_host.h +2 -2
Index: linux-stable-rt/arch/powerpc/include/asm/kvm_host.h
===================================================================
@@ -295,7 +295,7 @@
 	u8 in_guest;
 	struct list_head runnable_threads;
 	spinlock_t lock;
-	wait_queue_head_t wq;
+	struct swait_head wq;
 	u64 stolen_tb;
 	u64 preempt_tb;
 	struct kvm_vcpu *runner;

@@ -612,7 +612,7 @@
 	u8 prodded;
 	u32 last_inst;
 
-	wait_queue_head_t *wqp;
+	struct swait_head *wqp;
 	struct kvmppc_vcore *vcore;
 	int ret;
 	int trap;
arch/powerpc/kvm/book3s_hv.c +10 -10
Index: linux-stable-rt/arch/powerpc/kvm/book3s_hv.c
===================================================================
@@ -74,11 +74,11 @@
 {
 	int me;
 	int cpu = vcpu->cpu;
-	wait_queue_head_t *wqp;
+	struct swait_head *wqp;
 
 	wqp = kvm_arch_vcpu_wq(vcpu);
-	if (waitqueue_active(wqp)) {
-		wake_up_interruptible(wqp);
+	if (swaitqueue_active(wqp)) {
+		swait_wake_interruptible(wqp);
 		++vcpu->stat.halt_wakeup;
 	}
 

@@ -583,8 +583,8 @@
 		tvcpu->arch.prodded = 1;
 		smp_mb();
 		if (vcpu->arch.ceded) {
-			if (waitqueue_active(&vcpu->wq)) {
-				wake_up_interruptible(&vcpu->wq);
+			if (swaitqueue_active(&vcpu->wq)) {
+				swait_wake_interruptible(&vcpu->wq);
 				vcpu->stat.halt_wakeup++;
 			}
 		}

@@ -1199,7 +1199,7 @@
 		if (vcore) {
 			INIT_LIST_HEAD(&vcore->runnable_threads);
 			spin_lock_init(&vcore->lock);
-			init_waitqueue_head(&vcore->wq);
+			init_swait_head(&vcore->wq);
 			vcore->preempt_tb = TB_NIL;
 			vcore->lpcr = kvm->arch.lpcr;
 			vcore->first_vcpuid = core * threads_per_core;

@@ -1566,13 +1566,13 @@
  */
 static void kvmppc_vcore_blocked(struct kvmppc_vcore *vc)
 {
-	DEFINE_WAIT(wait);
+	DEFINE_SWAITER(wait);
 
-	prepare_to_wait(&vc->wq, &wait, TASK_INTERRUPTIBLE);
+	swait_prepare(&vc->wq, &wait, TASK_INTERRUPTIBLE);
 	vc->vcore_state = VCORE_SLEEPING;
 	spin_unlock(&vc->lock);
 	schedule();
-	finish_wait(&vc->wq, &wait);
+	swait_finish(&vc->wq, &wait);
 	spin_lock(&vc->lock);
 	vc->vcore_state = VCORE_INACTIVE;
 }

@@ -1613,7 +1613,7 @@
 			kvmppc_create_dtl_entry(vcpu, vc);
 			kvmppc_start_thread(vcpu);
 		} else if (vc->vcore_state == VCORE_SLEEPING) {
-			wake_up(&vc->wq);
+			swait_wake(&vc->wq);
 		}
 
 	}
arch/s390/include/asm/kvm_host.h +1 -1
Index: linux-stable-rt/arch/s390/include/asm/kvm_host.h
===================================================================
@@ -233,7 +233,7 @@
 	atomic_t active;
 	struct kvm_s390_float_interrupt *float_int;
 	int timer_due; /* event indicator for waitqueue below */
-	wait_queue_head_t *wq;
+	struct swait_head *wq;
 	atomic_t *cpuflags;
 	unsigned int action_bits;
 };
arch/s390/kvm/interrupt.c +10 -12
Index: linux-stable-rt/arch/s390/kvm/interrupt.c
===================================================================
@@ -403,7 +403,7 @@
 int kvm_s390_handle_wait(struct kvm_vcpu *vcpu)
 {
 	u64 now, sltime;
-	DECLARE_WAITQUEUE(wait, current);
+	DECLARE_SWAITER(wait);
 
 	vcpu->stat.exit_wait_state++;
 	if (kvm_cpu_has_interrupt(vcpu))

@@ -440,12 +440,11 @@
 	srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
 	spin_lock(&vcpu->arch.local_int.float_int->lock);
 	spin_lock_bh(&vcpu->arch.local_int.lock);
-	add_wait_queue(&vcpu->wq, &wait);
+	swait_prepare(&vcpu->wq, &wait, TASK_INTERRUPTIBLE);
 	while (list_empty(&vcpu->arch.local_int.list) &&
 		list_empty(&vcpu->arch.local_int.float_int->list) &&
 		(!vcpu->arch.local_int.timer_due) &&
 		!signal_pending(current)) {
-		set_current_state(TASK_INTERRUPTIBLE);
 		spin_unlock_bh(&vcpu->arch.local_int.lock);
 		spin_unlock(&vcpu->arch.local_int.float_int->lock);
 		schedule();

@@ -453,8 +452,7 @@
 		spin_lock_bh(&vcpu->arch.local_int.lock);
 	}
 	__unset_cpu_idle(vcpu);
-	__set_current_state(TASK_RUNNING);
-	remove_wait_queue(&vcpu->wq, &wait);
+	swait_finish(&vcpu->wq, &wait);
 	spin_unlock_bh(&vcpu->arch.local_int.lock);
 	spin_unlock(&vcpu->arch.local_int.float_int->lock);
 	vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);

@@ -469,8 +467,8 @@
 
 	spin_lock(&vcpu->arch.local_int.lock);
 	vcpu->arch.local_int.timer_due = 1;
-	if (waitqueue_active(&vcpu->wq))
-		wake_up_interruptible(&vcpu->wq);
+	if (swaitqueue_active(&vcpu->wq))
+		swait_wake_interruptible(&vcpu->wq);
 	spin_unlock(&vcpu->arch.local_int.lock);
 }
 

@@ -617,7 +615,7 @@
 	spin_lock_bh(&li->lock);
 	list_add(&inti->list, &li->list);
 	atomic_set(&li->active, 1);
-	BUG_ON(waitqueue_active(li->wq));
+	BUG_ON(swaitqueue_active(li->wq));
 	spin_unlock_bh(&li->lock);
 	return 0;
 }

@@ -750,8 +748,8 @@
 	li = fi->local_int[sigcpu];
 	spin_lock_bh(&li->lock);
 	atomic_set_mask(CPUSTAT_EXT_INT, li->cpuflags);
-	if (waitqueue_active(li->wq))
-		wake_up_interruptible(li->wq);
+	if (swaitqueue_active(li->wq))
+		swait_wake_interruptible(li->wq);
 	spin_unlock_bh(&li->lock);
 	spin_unlock(&fi->lock);
 	mutex_unlock(&kvm->lock);

@@ -836,8 +834,8 @@
 	if (inti->type == KVM_S390_SIGP_STOP)
 		li->action_bits |= ACTION_STOP_ON_STOP;
 	atomic_set_mask(CPUSTAT_EXT_INT, li->cpuflags);
-	if (waitqueue_active(&vcpu->wq))
-		wake_up_interruptible(&vcpu->wq);
+	if (swaitqueue_active(&vcpu->wq))
+		swait_wake_interruptible(&vcpu->wq);
 	spin_unlock_bh(&li->lock);
 	mutex_unlock(&vcpu->kvm->lock);
 	return 0;
arch/s390/kvm/sigp.c +8 -8
Index: linux-stable-rt/arch/s390/kvm/sigp.c
===================================================================
@@ -79,8 +79,8 @@
 	list_add_tail(&inti->list, &li->list);
 	atomic_set(&li->active, 1);
 	atomic_set_mask(CPUSTAT_EXT_INT, li->cpuflags);
-	if (waitqueue_active(li->wq))
-		wake_up_interruptible(li->wq);
+	if (swaitqueue_active(li->wq))
+		swait_wake_interruptible(li->wq);
 	spin_unlock_bh(&li->lock);
 	rc = SIGP_CC_ORDER_CODE_ACCEPTED;
 	VCPU_EVENT(vcpu, 4, "sent sigp emerg to cpu %x", cpu_addr);

@@ -148,8 +148,8 @@
 	list_add_tail(&inti->list, &li->list);
 	atomic_set(&li->active, 1);
 	atomic_set_mask(CPUSTAT_EXT_INT, li->cpuflags);
-	if (waitqueue_active(li->wq))
-		wake_up_interruptible(li->wq);
+	if (swaitqueue_active(li->wq))
+		swait_wake_interruptible(li->wq);
 	spin_unlock_bh(&li->lock);
 	rc = SIGP_CC_ORDER_CODE_ACCEPTED;
 	VCPU_EVENT(vcpu, 4, "sent sigp ext call to cpu %x", cpu_addr);

@@ -179,8 +179,8 @@
 	atomic_set(&li->active, 1);
 	atomic_set_mask(CPUSTAT_STOP_INT, li->cpuflags);
 	li->action_bits |= action;
-	if (waitqueue_active(li->wq))
-		wake_up_interruptible(li->wq);
+	if (swaitqueue_active(li->wq))
+		swait_wake_interruptible(li->wq);
 out:
 	spin_unlock_bh(&li->lock);
 

@@ -288,8 +288,8 @@
 
 	list_add_tail(&inti->list, &li->list);
 	atomic_set(&li->active, 1);
-	if (waitqueue_active(li->wq))
-		wake_up_interruptible(li->wq);
+	if (swaitqueue_active(li->wq))
+		swait_wake_interruptible(li->wq);
 	rc = SIGP_CC_ORDER_CODE_ACCEPTED;
 
 	VCPU_EVENT(vcpu, 4, "set prefix of cpu %02x to %x", cpu_addr, address);
arch/x86/kvm/lapic.c +3 -3
Index: linux-stable-rt/arch/x86/kvm/lapic.c
===================================================================
@@ -1533,7 +1533,7 @@
 	struct kvm_timer *ktimer = container_of(data, struct kvm_timer, timer);
 	struct kvm_lapic *apic = container_of(ktimer, struct kvm_lapic, lapic_timer);
 	struct kvm_vcpu *vcpu = apic->vcpu;
-	wait_queue_head_t *q = &vcpu->wq;
+	struct swait_head *q = &vcpu->wq;
 
 	/*
 	 * There is a race window between reading and incrementing, but we do

@@ -1547,8 +1547,8 @@
 		kvm_make_request(KVM_REQ_PENDING_TIMER, vcpu);
 	}
 
-	if (waitqueue_active(q))
-		wake_up_interruptible(q);
+	if (swaitqueue_active(q))
+		swait_wake_interruptible(q);
 
 	if (lapic_is_periodic(apic)) {
 		hrtimer_add_expires_ns(&ktimer->timer, ktimer->period);
include/linux/kvm_host.h +2 -2
Index: linux-stable-rt/include/linux/kvm_host.h
===================================================================
@@ -231,7 +231,7 @@
 
 	int fpu_active;
 	int guest_fpu_loaded, guest_xcr0_loaded;
-	wait_queue_head_t wq;
+	struct swait_head wq;
 	struct pid *pid;
 	int sigset_active;
 	sigset_t sigset;

@@ -677,7 +677,7 @@
 }
 #endif
 
-static inline wait_queue_head_t *kvm_arch_vcpu_wq(struct kvm_vcpu *vcpu)
+static inline struct swait_head *kvm_arch_vcpu_wq(struct kvm_vcpu *vcpu)
 {
 #ifdef __KVM_HAVE_ARCH_WQP
 	return vcpu->arch.wqp;
virt/kvm/async_pf.c +2 -2
Index: linux-stable-rt/virt/kvm/async_pf.c
===================================================================
@@ -82,8 +82,8 @@
 
 	trace_kvm_async_pf_completed(addr, gva);
 
-	if (waitqueue_active(&vcpu->wq))
-		wake_up_interruptible(&vcpu->wq);
+	if (swaitqueue_active(&vcpu->wq))
+		swait_wake_interruptible(&vcpu->wq);
 
 	mmput(mm);
 	kvm_put_kvm(vcpu->kvm);
virt/kvm/kvm_main.c +8 -8
Index: linux-stable-rt/virt/kvm/kvm_main.c
===================================================================
@@ -219,7 +219,7 @@
 	vcpu->kvm = kvm;
 	vcpu->vcpu_id = id;
 	vcpu->pid = NULL;
-	init_waitqueue_head(&vcpu->wq);
+	init_swait_head(&vcpu->wq);
 	kvm_async_pf_vcpu_init(vcpu);
 
 	page = alloc_page(GFP_KERNEL | __GFP_ZERO);

@@ -1675,10 +1675,10 @@
  */
 void kvm_vcpu_block(struct kvm_vcpu *vcpu)
 {
-	DEFINE_WAIT(wait);
+	DEFINE_SWAITER(wait);
 
 	for (;;) {
-		prepare_to_wait(&vcpu->wq, &wait, TASK_INTERRUPTIBLE);
+		swait_prepare(&vcpu->wq, &wait, TASK_INTERRUPTIBLE);
 
 		if (kvm_arch_vcpu_runnable(vcpu)) {
 			kvm_make_request(KVM_REQ_UNHALT, vcpu);

@@ -1692,7 +1692,7 @@
 		schedule();
 	}
 
-	finish_wait(&vcpu->wq, &wait);
+	swait_finish(&vcpu->wq, &wait);
 }
 EXPORT_SYMBOL_GPL(kvm_vcpu_block);
 

@@ -1704,11 +1704,11 @@
 {
 	int me;
 	int cpu = vcpu->cpu;
-	wait_queue_head_t *wqp;
+	struct swait_head *wqp;
 
 	wqp = kvm_arch_vcpu_wq(vcpu);
-	if (waitqueue_active(wqp)) {
-		wake_up_interruptible(wqp);
+	if (swaitqueue_active(wqp)) {
+		swait_wake_interruptible(wqp);
 		++vcpu->stat.halt_wakeup;
 	}
 

@@ -1814,7 +1814,7 @@
 				continue;
 			if (vcpu == me)
 				continue;
-			if (waitqueue_active(&vcpu->wq))
+			if (swaitqueue_active(&vcpu->wq))
 				continue;
 			if (!kvm_vcpu_eligible_for_directed_yield(vcpu))
 				continue;
#2 lore raw

Re: [patch -rt 1/2] KVM: use simple waitqueue for vcpu->wq

Quoted 14 lines · Marcelo Tosatti

The problem:

On -RT, an emulated LAPIC timer instances has the following path:

1) hard interrupt
2) ksoftirqd is scheduled
3) ksoftirqd wakes up vcpu thread
4) vcpu thread is scheduled

This extra context switch introduces unnecessary latency in the LAPIC path for a KVM guest.

The solution:

Quoted 8 lines

Allow waking up vcpu thread from hardirq context, thus avoiding the need for ksoftirqd to be scheduled.

Normal waitqueues make use of spinlocks, which on -RT are sleepable locks. Therefore, waking up a waitqueue waiter involves locking a sleeping lock, which is not allowed from hard interrupt context.

What are the consequences for the realtime kernel of making waitqueue traversal non-preemptible?

Is waitqueue traversal something that ever takes so long we need to care about it being non-preemptible?

Acked-by
#3 lore raw

Re: [patch -rt 1/2] KVM: use simple waitqueue for vcpu->wq

Quoted 24 lines · Rik van Riel
Quoted 14 lines · Marcelo Tosatti

The problem:

On -RT, an emulated LAPIC timer instances has the following path:

1) hard interrupt
2) ksoftirqd is scheduled
3) ksoftirqd wakes up vcpu thread
4) vcpu thread is scheduled

This extra context switch introduces unnecessary latency in the LAPIC path for a KVM guest.

The solution:

Quoted 8 lines

Allow waking up vcpu thread from hardirq context, thus avoiding the need for ksoftirqd to be scheduled.

Normal waitqueues make use of spinlocks, which on -RT are sleepable locks. Therefore, waking up a waitqueue waiter involves locking a sleeping lock, which is not allowed from hard interrupt context.

Quoted 5 lines

What are the consequences for the realtime kernel of making waitqueue traversal non-preemptible?

Is waitqueue traversal something that ever takes so long we need to care about it being non-preemptible?

I answered my own question.

This patch only changes the kvm vcpu waitqueue, which should only ever have the vcpu thread itself waiting on it. In other words, it is a wait "queue" of just one entry long, and the latency of traversing it will be absolutely minimal.

Unless someone can think of a better way to implement this patch, it gets my:

Acked-by Rik van Riel <[email protected]>
author
#4 lore raw

Re: [patch -rt 1/2] KVM: use simple waitqueue for vcpu->wq

In reply to Rik van Riel
Quoted 24 lines · Rik van Riel
Quoted 14 lines · Marcelo Tosatti

The problem:

On -RT, an emulated LAPIC timer instances has the following path:

1) hard interrupt
2) ksoftirqd is scheduled
3) ksoftirqd wakes up vcpu thread
4) vcpu thread is scheduled

This extra context switch introduces unnecessary latency in the LAPIC path for a KVM guest.

The solution:

Quoted 8 lines

Allow waking up vcpu thread from hardirq context, thus avoiding the need for ksoftirqd to be scheduled.

Normal waitqueues make use of spinlocks, which on -RT are sleepable locks. Therefore, waking up a waitqueue waiter involves locking a sleeping lock, which is not allowed from hard interrupt context.

Quoted 5 lines

What are the consequences for the realtime kernel of making waitqueue traversal non-preemptible?

Is waitqueue traversal something that ever takes so long we need to care about it being non-preemptible?

commit d872cfa018625071a3a6b1ea8a62a2790d2c157a

Author: Thomas Gleixner <[email protected]>
Date:   Mon Dec 12 12:29:04 2011 +0100

wait-simple: Simple waitqueue implementation

wait_queue is a swiss army knife and in most of the cases the complexity is not needed. For RT waitqueues are a constant source of trouble as we can't convert the head lock to a raw spinlock due to fancy and long lasting callbacks.

Unsure about the details...

#5 lore raw

Re: [patch -rt 1/2] KVM: use simple waitqueue for vcpu->wq

In reply to Rik van Riel
Quoted 24 lines · Rik van Riel
Quoted 14 lines · Marcelo Tosatti

The problem:

On -RT, an emulated LAPIC timer instances has the following path:

1) hard interrupt
2) ksoftirqd is scheduled
3) ksoftirqd wakes up vcpu thread
4) vcpu thread is scheduled

This extra context switch introduces unnecessary latency in the LAPIC path for a KVM guest.

The solution:

Quoted 8 lines

Allow waking up vcpu thread from hardirq context, thus avoiding the need for ksoftirqd to be scheduled.

Normal waitqueues make use of spinlocks, which on -RT are sleepable locks. Therefore, waking up a waitqueue waiter involves locking a sleeping lock, which is not allowed from hard interrupt context.

Quoted 5 lines

What are the consequences for the realtime kernel of making waitqueue traversal non-preemptible?

Is waitqueue traversal something that ever takes so long we need to care about it being non-preemptible?

__wake_up

	lock_irq_save()
	__wake_up_common()
		for_each_entry(curr)
			curr->func()
	unlock_irq_save()

There are two issues here:

1) Long waiter chains. We probably could work around that in some creative way

2) The non default callbacks. default is default_wake_function.

The non default callbacks, which are used by nfsd and other stuff are calling the world and some more and take preemptible locks and do other fancy stuff which falls flat on its nose when called under a raw lock on rt

So I created the swait replacement which has a smaller memory footprint, less featuritis and a raw lock because its only wakes up, no custom callbacks.

The still suffer #1, but most use cases on RT are single waiter scenarios anyway.

Hope that helps.

Thanks,

tglx

author
#6 lore raw

PATCH 2/2 KVM: lapic: mark LAPIC timer handler as irqsafe

In reply to Marcelo Tosatti

Since lapic timer handler only wakes up a simple waitqueue, it can be executed from hardirq context.

Reduces average cyclictest latency by 3us.

Signed-off-by Marcelo Tosatti <[email protected]>
---
 arch/x86/kvm/lapic.c |   42 +++++++++++++++++++++++++++++++++++++++---
 1 file changed, 39 insertions(+), 3 deletions(-)
arch/x86/kvm/lapic.c +39 -3
Index: linux-stable-rt/arch/x86/kvm/lapic.c
===================================================================
@@ -1031,8 +1031,38 @@
 				   apic->divide_count);
 }
 
+
+static enum hrtimer_restart apic_timer_fn(struct hrtimer *data);
+
+static void apic_timer_expired(struct hrtimer *data)
+{
+	int ret, i = 0;
+	enum hrtimer_restart r;
+	struct kvm_timer *ktimer = container_of(data, struct kvm_timer, timer);
+
+	r = apic_timer_fn(data);
+
+	if (r == HRTIMER_RESTART) {
+		do {
+			ret = hrtimer_start_expires(data, HRTIMER_MODE_ABS);
+			if (ret == -ETIME)
+				hrtimer_add_expires_ns(&ktimer->timer,
+							ktimer->period);
+			i++;
+		} while (ret == -ETIME && i < 10);
+
+		if (ret == -ETIME) {
+			printk(KERN_ERR "%s: failed to reprogram timer\n",
+			       __func__);
+			WARN_ON(1);
+		}
+	}
+}
+
+
 static void start_apic_timer(struct kvm_lapic *apic)
 {
+	int ret;
 	ktime_t now;
 	atomic_set(&apic->lapic_timer.pending, 0);
 

@@ -1062,9 +1092,11 @@
 			}
 		}
 
-		hrtimer_start(&apic->lapic_timer.timer,
+		ret = hrtimer_start(&apic->lapic_timer.timer,
 			      ktime_add_ns(now, apic->lapic_timer.period),
 			      HRTIMER_MODE_ABS);
+		if (ret == -ETIME)
+			apic_timer_expired(&apic->lapic_timer.timer);
 
 		apic_debug("%s: bus cycle is %" PRId64 "ns, now 0x%016"
 			   PRIx64 ", "

@@ -1094,8 +1126,10 @@
 			ns = (tscdeadline - guest_tsc) * 1000000ULL;
 			do_div(ns, this_tsc_khz);
 		}
-		hrtimer_start(&apic->lapic_timer.timer,
+		ret = hrtimer_start(&apic->lapic_timer.timer,
 			ktime_add_ns(now, ns), HRTIMER_MODE_ABS);
+		if (ret == -ETIME)
+			apic_timer_expired(&apic->lapic_timer.timer);
 
 		local_irq_restore(flags);
 	}

@@ -1581,6 +1615,7 @@
 	hrtimer_init(&apic->lapic_timer.timer, CLOCK_MONOTONIC,
 		     HRTIMER_MODE_ABS);
 	apic->lapic_timer.timer.function = apic_timer_fn;
+	apic->lapic_timer.timer.irqsafe = 1;
 
 	/*
 	 * APIC is created enabled. This will prevent kvm_lapic_set_base from

@@ -1699,7 +1734,8 @@
 
 	timer = &vcpu->arch.apic->lapic_timer.timer;
 	if (hrtimer_cancel(timer))
-		hrtimer_start_expires(timer, HRTIMER_MODE_ABS);
+		if (hrtimer_start_expires(timer, HRTIMER_MODE_ABS) == -ETIME)
+			apic_timer_expired(timer);
 }
 
 /*
maintainer
#7 lore raw

Re: [patch -rt 2/2] KVM: lapic: mark LAPIC timer handler as irqsafe

arch/x86/kvm/lapic.c line 1050

Quoted 7 lines · Marcelo Tosatti
arch/x86/kvm/lapic.c +7 -0
+
+	if (r == HRTIMER_RESTART) {
+		do {
+			ret = hrtimer_start_expires(data, HRTIMER_MODE_ABS);
+			if (ret == -ETIME)
+				hrtimer_add_expires_ns(&ktimer->timer,
+							ktimer->period);

Is it possible to just compute the time where the next interrupt happens? I suspect the printk and WARN_ON below can be easily triggered by a guest.

Paolo

Quoted 9 lines
arch/x86/kvm/lapic.c +9 -0
+			i++;
+		} while (ret == -ETIME && i < 10);
+
+		if (ret == -ETIME) {
+			printk(KERN_ERR "%s: failed to reprogram timer\n",
+			       __func__);
+			WARN_ON(1);
+		}
+	}
#8 lore raw

Re: [patch -rt 2/2] KVM: lapic: mark LAPIC timer handler as irqsafe

Quoted 14 lines · Paolo Bonzini

arch/x86/kvm/lapic.c line 1050

Quoted 7 lines · Marcelo Tosatti
arch/x86/kvm/lapic.c +7 -0
+
+	if (r == HRTIMER_RESTART) {
+		do {
+			ret = hrtimer_start_expires(data, HRTIMER_MODE_ABS);
+			if (ret == -ETIME)
+				hrtimer_add_expires_ns(&ktimer->timer,
+							ktimer->period);

Is it possible to just compute the time where the next interrupt happens? I suspect the printk and WARN_ON below can be easily triggered by a guest.

We have a lower bound for the period that a guest can program. Unless that value is set too low, this should practically not happen if we avoid disturbances while handling the event and reprogramming the next one (irqs off?).

Jan

Signature · 2 lines
Siemens AG, Corporate Technology, CT RTC ITP SES-DE
Corporate Competence Center Embedded Linux
author
#9 lore raw

Re: [patch -rt 2/2] KVM: lapic: mark LAPIC timer handler as irqsafe

In reply to Paolo Bonzini
Quoted 13 lines · Paolo Bonzini

arch/x86/kvm/lapic.c line 1050

Quoted 7 lines · Marcelo Tosatti
arch/x86/kvm/lapic.c +7 -0
+
+	if (r == HRTIMER_RESTART) {
+		do {
+			ret = hrtimer_start_expires(data, HRTIMER_MODE_ABS);
+			if (ret == -ETIME)
+				hrtimer_add_expires_ns(&ktimer->timer,
+							ktimer->period);

Is it possible to just compute the time where the next interrupt happens?

Yes but there is no guarantee hrtimer_start_expires will not fail.

Quoted 2 lines

I suspect the printk and WARN_ON below can be easily triggered by a guest.

I'll remove those, thanks.

Acked-by on v1 2/2
#10 lore raw

Re: [patch -rt 2/2] KVM: lapic: mark LAPIC timer handler as irqsafe

In reply to Marcelo Tosatti
Quoted 4 lines · Marcelo Tosatti

Since lapic timer handler only wakes up a simple waitqueue, it can be executed from hardirq context.

Reduces average cyclictest latency by 3us.

Can this patch be merged in the KVM tree, and go
upstream via Paolo?

Quoted 1 line
Signed-off-by Marcelo Tosatti <[email protected]>
Acked-by Rik van Riel <[email protected]>
#11 lore raw

Re: [patch -rt 2/2] KVM: lapic: mark LAPIC timer handler as irqsafe

Quoted 8 lines · Rik van Riel
Quoted 4 lines · Marcelo Tosatti

Since lapic timer handler only wakes up a simple waitqueue, it can be executed from hardirq context.

Reduces average cyclictest latency by 3us.

Can this patch be merged in the KVM tree, and go
upstream via Paolo?

Not really as it has RT dependencies ....

Thanks,

tglx

maintainer
#12 lore raw

Re: [patch -rt 2/2] KVM: lapic: mark LAPIC timer handler as irqsafe

Quoted 11 lines · Thomas Gleixner
Quoted 8 lines · Rik van Riel
Quoted 4 lines · Marcelo Tosatti

Since lapic timer handler only wakes up a simple waitqueue, it can be executed from hardirq context.

Reduces average cyclictest latency by 3us.

Can this patch be merged in the KVM tree, and go
upstream via Paolo?

Not really as it has RT dependencies ....

Can hrtimer_start_expires even return ETIME on a non-RT kernel? If yes, I can take patch 2. If not, it's better to keep both patches in the RT tree.

Paolo

author
#13 lore raw

Re: [patch -rt 2/2] KVM: lapic: mark LAPIC timer handler as irqsafe

Quoted 13 lines · Paolo Bonzini
Quoted 10 lines · Thomas Gleixner
Quoted 8 lines · Rik van Riel
Quoted 4 lines · Marcelo Tosatti

Since lapic timer handler only wakes up a simple waitqueue, it can be executed from hardirq context.

Reduces average cyclictest latency by 3us.

Can this patch be merged in the KVM tree, and go
upstream via Paolo?

Quoted 7 lines
Quoted 1 line

Not really as it has RT dependencies ....

Can hrtimer_start_expires even return ETIME on a non-RT kernel? If yes, I can take patch 2. If not, it's better to keep both patches in the RT tree.

Paolo

It can't. We're still evaluating the necessity of this patch, will resubmit accordingly.