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Subject[PATCH RFC V4 3/3] kvm: Choose better candidate for directed yield
From: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>

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 <raghavendra.kt@linux.vnet.ibm.com>
---
V2 was:
Reviewed-by: Rik van Riel <riel@redhat.com>
include/linux/kvm_host.h | 5 +++++
virt/kvm/kvm_main.c | 35 +++++++++++++++++++++++++++++++++++
2 files changed, 40 insertions(+), 0 deletions(-)
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index fa353bd..cfb38a2 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -925,6 +925,11 @@ static inline void kvm_vcpu_set_dy_eligible(struct kvm_vcpu *vcpu, bool val)
{
}

+static inline bool kvm_vcpu_check_and_update_eligible(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 c50213f..6a82580 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 @cpu_relax_intercepted.
+ * 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_check_and_update_eligible(struct kvm_vcpu *vcpu)
+{
+ bool eligible;
+
+ eligible = !vcpu->ple.cpu_relax_intercepted ||
+ (vcpu->ple.cpu_relax_intercepted &&
+ vcpu->ple.dy_eligible);
+
+ if (vcpu->ple.cpu_relax_intercepted)
+ vcpu->ple.dy_eligible = !vcpu->ple.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_check_and_update_eligible(vcpu))
+ continue;
if (kvm_vcpu_yield_to(vcpu)) {
kvm->last_boosted_vcpu = i;
yielded = 1;


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