[PATCH v3 2/5] KVM: X86: Bail out of direct yield in case of under-committed scenarios

[Wanpeng Li <kernellwp@gmail.com>]

From: Wanpeng Li <wanpengli@tencent.com>

In case of under-committed scenarios, vCPU can get scheduling easily,
kvm_vcpu_yield_to add 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 in such scenarios by checking the length
of current cpu runqueue, it can be treated as a hint of under-committed
instead of guarantee of accuracy. The directed_yield_successful/attempted 
ratio can be improved from 50+% to 80+% in the under-committed scenario. 

Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
v2 -> v3:
 * update patch description
v1 -> v2:
 * move the check after attempted counting
 * update patch description

 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 9b6bca616929..dfb7c320581f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8360,6 +8360,9 @@ static void kvm_sched_yield(struct kvm_vcpu *vcpu, unsigned long dest_id)
 
 	vcpu->stat.directed_yield_attempted++;
 
+	if (single_task_running())
+		goto no_yield;
+
 	rcu_read_lock();
 	map = rcu_dereference(vcpu->kvm->arch.apic_map);
 
-- 
2.25.1