linux-kernel - Re: [PATCH v2 2/4] KVM: X86: Bail out of direct yield in case of under-comitted scenarios

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Date:   Mon, 17 May 2021 09:03:47 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     David Matlack <dmatlack@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        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: Re: [PATCH v2 2/4] KVM: X86: Bail out of direct yield in case of
 under-comitted scenarios

On Sat, 15 May 2021 at 05:33, David Matlack <dmatlack@...gle.com> wrote:
>
> On Wed, May 12, 2021 at 7:01 PM Wanpeng Li <kernellwp@...il.com> wrote:
> >
> > From: Wanpeng Li <wanpengli@...cent.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@...cent.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;
>
> Since this is a heuristic, do you have any experimental or real world
> results that show the benefit?

I observe the directed_yield_successful/directed_yield_attempted
ratio, it can be improved from 50%+ to 80%+ in the under-committed
scenario.

     Wanpeng