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

[Paolo Bonzini]

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.

Paolo