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Message-ID: <6d9e85ac5768e920805f121eeaff1360f3b257df.camel@suse.com>
Subject: Re: [PATCH] KVM: Disable wake-affine vCPU process to mitigate lock
 holder preemption
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 =?UTF-8?Q?Kr=C4=8Dm=C3=A1=C5=99?= <rkrcmar@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>
Date:   Thu, 01 Aug 2019 14:39:34 +0200
In-Reply-To: <19e0beb6-a732-ea1f-79a5-41be92569338@redhat.com>
References: <1564479235-25074-1-git-send-email-wanpengli@tencent.com>
         <19e0beb6-a732-ea1f-79a5-41be92569338@redhat.com>
Organization: SUSE
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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=20
> > sleeping vCPU, it increases the probability to stack vCPUs/qemu by
> > scheduler
> > wake-affine. vCPU stacking issue can greately inceases the lock
> > synchronization=20
> > latency in a virtualized environment. This patch disables wake-
> > affine vCPU=20
> > process to mitigtate lock holder preemption.
>=20
> There is no guarantee that the vCPU remains on the thread where it's
> created, so the patch is not enough.
>=20
> If many vCPUs are stacked on the same pCPU, why doesn't the wake_cap
> kick in sooner or later?
>=20
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
--=20
Dario Faggioli, Ph.D
http://about.me/dario.faggioli
Virtualization Software Engineer
SUSE Labs, SUSE https://www.suse.com/
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