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

[Wanpeng Li]

On Wed, 19 May 2021 at 03:21, Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Tue, May 18, 2021, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> >
> > 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. "