[RFC,V5,3/3] kvm: Choose better candidate for directed yield

Commit Message

Raghavendra K T July 18, 2012, 1:38 p.m. UTC

From: Raghavendra K T <[e-mail]>

Currently, on a large vcpu guests, there is a high probability of
yielding to the same vcpu who had recently done a pause-loop exit or
cpu relax intercepted. Such a yield can lead to the vcpu spinning
again and hence degrade the performance.

The patchset keeps track of the pause loop exit/cpu relax interception
and gives chance to a vcpu which:
 (a) Has not done pause loop exit or cpu relax intercepted at all
     (probably he is preempted lock-holder)
 (b) Was skipped in last iteration because it did pause loop exit or
     cpu relax intercepted, and probably has become eligible now
     (next eligible lock holder)

Signed-off-by: Raghavendra K T <[e-mail]>
---
V2 was:
Reviewed-by: Rik van Riel <[e-mail]>

 include/linux/kvm_host.h |    5 +++++
 virt/kvm/kvm_main.c      |   36 ++++++++++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+), 0 deletions(-)


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Comments

Raghavendra K T July 18, 2012, 2:39 p.m. UTC | #1

On 07/18/2012 07:08 PM, Raghavendra K T wrote:
> From: Raghavendra K T<[e-mail]>
> +bool kvm_vcpu_eligible_for_directed_yield(struct kvm_vcpu *vcpu)
> +{
> +	bool eligible;
> +
> +	eligible = !vcpu->spin_loop.in_spin_loop ||
> +			(vcpu->spin_loop.in_spin_loop&&
> +			 vcpu->spin_loop.dy_eligible);
> +
> +	if (vcpu->spin_loop.in_spin_loop)
> +		vcpu->spin_loop.dy_eligible = !vcpu->spin_loop.dy_eligible;
> +
> +	return eligible;
> +}

I should have added a comment like:
Since algorithm is based on heuristics, accessing another vcpu data
without locking does not harm. It may result in trying to yield to  same 
VCPU, fail and continue with next and so on.

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Patch

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 34ce296..952427d 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -923,6 +923,11 @@  static inline void kvm_vcpu_set_dy_eligible(struct kvm_vcpu *vcpu, bool val)
 {
 }
 
+static inline bool kvm_vcpu_eligible_for_directed_yield(struct kvm_vcpu *vcpu)
+{
+	return true;
+}
+
 #endif /* CONFIG_HAVE_KVM_CPU_RELAX_INTERCEPT */
 #endif
 
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 3d6ffc8..bf9fb97 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1571,6 +1571,39 @@  bool kvm_vcpu_yield_to(struct kvm_vcpu *target)
 }
 EXPORT_SYMBOL_GPL(kvm_vcpu_yield_to);
 
+#ifdef CONFIG_HAVE_KVM_CPU_RELAX_INTERCEPT
+/*
+ * Helper that checks whether a VCPU is eligible for directed yield.
+ * Most eligible candidate to yield is decided by following heuristics:
+ *
+ *  (a) VCPU which has not done pl-exit or cpu relax intercepted recently
+ *  (preempted lock holder), indicated by @in_spin_loop.
+ *  Set at the beiginning and cleared at the end of interception/PLE handler.
+ *
+ *  (b) VCPU which has done pl-exit/ cpu relax intercepted but did not get
+ *  chance last time (mostly it has become eligible now since we have probably
+ *  yielded to lockholder in last iteration. This is done by toggling
+ *  @dy_eligible each time a VCPU checked for eligibility.)
+ *
+ *  Yielding to a recently pl-exited/cpu relax intercepted VCPU before yielding
+ *  to preempted lock-holder could result in wrong VCPU selection and CPU
+ *  burning. Giving priority for a potential lock-holder increases lock
+ *  progress.
+ */
+bool kvm_vcpu_eligible_for_directed_yield(struct kvm_vcpu *vcpu)
+{
+	bool eligible;
+
+	eligible = !vcpu->spin_loop.in_spin_loop ||
+			(vcpu->spin_loop.in_spin_loop &&
+			 vcpu->spin_loop.dy_eligible);
+
+	if (vcpu->spin_loop.in_spin_loop)
+		vcpu->spin_loop.dy_eligible = !vcpu->spin_loop.dy_eligible;
+
+	return eligible;
+}
+#endif
 void kvm_vcpu_on_spin(struct kvm_vcpu *me)
 {
 	struct kvm *kvm = me->kvm;
@@ -1599,6 +1632,8 @@  void kvm_vcpu_on_spin(struct kvm_vcpu *me)
 				continue;
 			if (waitqueue_active(&vcpu->wq))
 				continue;
+			if (!kvm_vcpu_eligible_for_directed_yield(vcpu))
+				continue;
 			if (kvm_vcpu_yield_to(vcpu)) {
 				kvm->last_boosted_vcpu = i;
 				yielded = 1;
@@ -1607,6 +1642,7 @@  void kvm_vcpu_on_spin(struct kvm_vcpu *me)
 		}
 	}
 	kvm_vcpu_set_in_spin_loop(me, false);
+	kvm_vcpu_set_dy_eligible(me, false);
 }
 EXPORT_SYMBOL_GPL(kvm_vcpu_on_spin);