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

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Message-ID: <20100603151558.GD4166@basil.fritz.box>
Date:	Thu, 3 Jun 2010 17:15:58 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	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 Thu, Jun 03, 2010 at 12:06:39PM +0100, David Woodhouse wrote:
> 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?

The problem is fitting the state into the u32

Also "lightly contended" is not that uncommon.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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