linux-kernel - Re: [PATCH] use unfair spinlock when running on hypervisor.

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Message-ID: <1275563199.2383.1873.camel@macbook.infradead.org>
Date:	Thu, 03 Jun 2010 12:06:39 +0100
From:	David Woodhouse <dwmw2@...radead.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	john cooper <john.cooper@...rd-harmonic.com>,
	Avi Kivity <avi@...hat.com>, Gleb Natapov <gleb@...hat.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, hpa@...or.com,
	mingo@...e.hu, npiggin@...e.de, tglx@...utronix.de,
	mtosatti@...hat.com, john cooper <john.cooper@...hat.com>
Subject: Re: [PATCH] use unfair spinlock when running on hypervisor.

On Tue, 2010-06-01 at 21:36 +0200, Andi Kleen wrote:
> > Collecting the contention/usage statistics on a per spinlock
> > basis seems complex.  I believe a practical approximation
> > to this are adaptive mutexes where upon hitting a spin
> > time threshold, punt and let the scheduler reconcile fairness.
> 
> That would probably work, except: how do you get the
> adaptive spinlock into a paravirt op without slowing
> down a standard kernel?

It only ever comes into play in the case where the spinlock is contended
anyway -- surely it shouldn't be _that_ much of a performance issue?

See the way that ppc64 handles it -- on a machine with overcommitted
virtual cpus, it will call __spin_yield (arch/powerpc/lib/locks.c) on
contention, which may cause the virtual CPU to donate its hypervisor
timeslice to the vCPU which is actually holding the lock in question.

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@...el.com                              Intel Corporation

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