From: Dario Faggioli <dfaggioli@suse.com>
To: Paolo Bonzini <pbonzini@redhat.com>,
Wanpeng Li <kernellwp@gmail.com>,
linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Cc: "Radim Krčmář" <rkrcmar@redhat.com>,
"Peter Zijlstra" <peterz@infradead.org>,
"Thomas Gleixner" <tglx@linutronix.de>
Subject: Re: [PATCH] KVM: Disable wake-affine vCPU process to mitigate lock holder preemption
Date: Thu, 01 Aug 2019 14:39:34 +0200 [thread overview]
Message-ID: <6d9e85ac5768e920805f121eeaff1360f3b257df.camel@suse.com> (raw)
In-Reply-To: <19e0beb6-a732-ea1f-79a5-41be92569338@redhat.com>
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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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next prev parent reply other threads:[~2019-08-01 12:39 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-30 9:33 [PATCH] KVM: Disable wake-affine vCPU process to mitigate lock holder preemption Wanpeng Li
2019-07-30 11:46 ` Paolo Bonzini
2019-08-01 12:39 ` Dario Faggioli [this message]
2019-07-30 12:09 ` Peter Zijlstra
2019-08-01 12:57 ` Dario Faggioli
2019-08-02 0:51 ` Wanpeng Li
2019-08-02 8:30 ` Christophe de Dinechin
2019-08-02 8:38 ` Paolo Bonzini
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