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From:   Wanpeng Li <kernellwp@gmail.com>
To:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>
Subject: [PATCH v2 2/4] KVM: X86: Bail out of direct yield in case of under-comitted scenarios
Date:   Thu, 13 May 2021 09:59:47 +0800
Message-Id: <1620871189-4763-1-git-send-email-wanpengli@tencent.com>
Precedence: bulk

From: Wanpeng Li <wanpengli@tencent.com>

In case of under-comitted 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 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.

Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
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 9b6bca6..dfb7c32 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);
 
-- 
2.7.4