[PATCH 2/3] KVM: X86: Bail out of direct yield in case of undercomitted scenarios

[Wanpeng Li]

From: Wanpeng Li <wanpengli@xxxxxxxxxxx>

In case of undercomitted scenarios, vCPU can get scheduling easily, 
kvm_vcpu_yield_to adds extra overhead, we can observe a lot of race 
between vcpu->ready is true and yield fails due to p->state is 
TASK_RUNNING. Let's bail out is such scenarios by checking the length 
of current cpu runqueue.

Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
---
 arch/x86/kvm/x86.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 5bd550e..c0244a6 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8358,6 +8358,9 @@ static void kvm_sched_yield(struct kvm_vcpu *vcpu, unsigned long dest_id)
 	struct kvm_vcpu *target = NULL;
 	struct kvm_apic_map *map;
 
+	if (single_task_running())
+		goto no_yield;
+
 	vcpu->stat.directed_yield_attempted++;
 
 	rcu_read_lock();
-- 
2.7.4