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

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Message-ID: <889a0a43-0641-70ce-d2a5-ed71bd54e59c@redhat.com>
Date:   Mon, 24 May 2021 15:42:30 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     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 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