KVM: Choose better candidate for directed yield
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>
Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com> # on s390x
Signed-off-by: Avi Kivity <avi@redhat.com>
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 0892b75..1e10ebe 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1579,6 +1579,43 @@
}
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.
+ *
+ * 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 VCPU and so on.
+ */
+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)
+ kvm_vcpu_set_dy_eligible(vcpu, !vcpu->spin_loop.dy_eligible);
+
+ return eligible;
+}
+#endif
void kvm_vcpu_on_spin(struct kvm_vcpu *me)
{
struct kvm *kvm = me->kvm;
@@ -1607,6 +1644,8 @@
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;
@@ -1615,6 +1654,9 @@
}
}
kvm_vcpu_set_in_spin_loop(me, false);
+
+ /* Ensure vcpu is not eligible during next spinloop */
+ kvm_vcpu_set_dy_eligible(me, false);
}
EXPORT_SYMBOL_GPL(kvm_vcpu_on_spin);