From: Wanpeng Li <kernellwp@gmail.com>
To: Sean Christopherson <seanjc@google.com>
Cc: LKML <linux-kernel@vger.kernel.org>, kvm <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
Date: Wed, 19 May 2021 10:57:55 +0800 [thread overview]
Message-ID: <CANRm+Cy_D3cBBEYQ9ApKMNC6p0dpTBQYQXs+dv5vrFedVkOy2w@mail.gmail.com> (raw)
In-Reply-To: <YKQTx381CGPp7uZY@google.com>
On Wed, 19 May 2021 at 03:21, Sean Christopherson <seanjc@google.com> wrote:
>
> 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.
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. "
next prev parent reply other threads:[~2021-05-19 2:58 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-18 12:00 [PATCH v4 1/5] KVM: exit halt polling on need_resched() for both book3s and generic halt-polling Wanpeng Li
2021-05-18 12:00 ` [PATCH v4 2/5] KVM: X86: Bail out of direct yield in case of under-committed scenarios Wanpeng Li
2021-05-18 19:21 ` Sean Christopherson
2021-05-19 2:57 ` Wanpeng Li [this message]
2021-05-24 13:42 ` Paolo Bonzini
2021-05-24 13:47 ` Wanpeng Li
2021-05-18 12:00 ` [PATCH v4 3/5] KVM: X86: Fix vCPU preempted state from guest's point of view Wanpeng Li
2021-05-18 12:00 ` [PATCH v4 4/5] KVM: X86: hyper-v: Task srcu lock when accessing kvm_memslots() Wanpeng Li
2021-05-18 12:00 ` [PATCH v4 5/5] KVM: LAPIC: Narrow the timer latency between wait_lapic_expire and world switch Wanpeng Li
2021-05-18 19:13 ` Sean Christopherson
2021-05-24 13:46 ` [PATCH v4 1/5] KVM: exit halt polling on need_resched() for both book3s and generic halt-polling Paolo Bonzini
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