linux-kernel - Re: [PATCH] KVM: Disable wake-affine vCPU process to mitigate lock holder preemption

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Date:   Thu, 01 Aug 2019 14:39:34 +0200
From:   Dario Faggioli <dfaggioli@...e.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] KVM: Disable wake-affine vCPU process to mitigate lock
 holder preemption

On Tue, 2019-07-30 at 13:46 +0200, Paolo Bonzini wrote:
> On 30/07/19 11:33, Wanpeng Li wrote:
> > When qemu/other vCPU inject virtual interrupt to guest through
> > waking up one 
> > sleeping vCPU, it increases the probability to stack vCPUs/qemu by
> > scheduler
> > wake-affine. vCPU stacking issue can greately inceases the lock
> > synchronization 
> > latency in a virtualized environment. This patch disables wake-
> > affine vCPU 
> > process to mitigtate lock holder preemption.
> 
> There is no guarantee that the vCPU remains on the thread where it's
> created, so the patch is not enough.
> 
> If many vCPUs are stacked on the same pCPU, why doesn't the wake_cap
> kick in sooner or later?
> 
Assuming it actually is the case that vcpus *do* get stacked *and* that
wake_cap() *doesn't* kick in, maybe it could be because of this check?

        /* Minimum capacity is close to max, no need to abort wake_affine */
        if (max_cap - min_cap < max_cap >> 3)
                return 0;

Regards
-- 
Dario Faggioli, Ph.D
http://about.me/dario.faggioli
Virtualization Software Engineer
SUSE Labs, SUSE https://www.suse.com/
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