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

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Date:   Sat,  8 May 2021 17:31:49 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: [PATCH 2/3] KVM: X86: Bail out of direct yield in case of undercomitted scenarios

From: Wanpeng Li <wanpengli@...cent.com>

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@...cent.com>
---
 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