linux-kernel - Re: [PATCH v4 2/5] KVM: X86: Bail out of direct yield in case of under-committed scenarios

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Date:   Mon, 24 May 2021 21:47:07 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v4 2/5] KVM: X86: Bail out of direct yield in case of
 under-committed scenarios

On Mon, 24 May 2021 at 21:42, Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 19/05/21 04:57, Wanpeng Li wrote:
> > Looks good. Hope Paolo can update the patch description when applying.:)
> >
> > "In case of under-committed scenarios, vCPU can get scheduling easily,
> > kvm_vcpu_yield_to add extra overhead, we can observe a lot of races
> > 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 guaranteeing accuracy. 30%+ of directed-yield attempts can
> > avoid the expensive lookups in kvm_sched_yield() in an under-committed
> > scenario. "
> >
>
> Here is what I used:
>
>      In case of under-committed scenarios, vCPUs can be scheduled easily;
>      kvm_vcpu_yield_to adds extra overhead, and it is also common to see
>      when vcpu->ready is true but yield later failing due to p->state is
>      TASK_RUNNING.
>
>      Let's bail out in such scenarios by checking the length of current cpu
>      runqueue, which can be treated as a hint of under-committed instead of
>      guarantee of accuracy. 30%+ of directed-yield attempts can now avoid
>      the expensive lookups in kvm_sched_yield() in an under-committed scenario.

Thanks Paolo! :)
    Wanpeng