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Date:   Tue, 18 May 2021 19:21:43 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Wanpeng Li <kernellwp@gmail.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>
Subject: Re: [PATCH v4 2/5] KVM: X86: Bail out of direct yield in case of
 under-committed scenarios
Message-ID: <YKQTx381CGPp7uZY@google.com>
References: <1621339235-11131-1-git-send-email-wanpengli@tencent.com>
 <1621339235-11131-2-git-send-email-wanpengli@tencent.com>
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On Tue, May 18, 2021, Wanpeng Li wrote:
> 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.

The "50+% to 80+%" comment will be a bit confusing for future readers now that
the single_task_running() case counts as an attempt.  I think the new comment
would be something like "30%+ of directed-yield attempts can avoid the expensive
lookups in kvm_sched_yield() in an under-committed scenario."  That would also
provide the real justification, as bumping the success ratio isn't the true goal
of this path.

> 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
>