linux-kernel - Re: [PATCH 1/4, v2] x86: enlightenment for ticket spin locks

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Message-Id: <4C2B60FE0200007800008D3B@vpn.id2.novell.com>
Date:	Wed, 30 Jun 2010 14:21:34 +0100
From:	"Jan Beulich" <JBeulich@...ell.com>
To:	"Jeremy Fitzhardinge" <jeremy@...p.org>
Cc:	"mingo@...e.hu" <mingo@...e.hu>,
	"Peter Zijlstra" <peterz@...radead.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"Ky Srinivasan" <KSrinivasan@...ell.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hpa@...or.com" <hpa@...or.com>
Subject: Re: [PATCH 1/4, v2] x86: enlightenment for ticket spin locks -
	 base implementation

>>> On 30.06.10 at 14:53, Jeremy Fitzhardinge <jeremy@...p.org> wrote:
> On 06/30/2010 01:52 PM, Jan Beulich wrote:
>> I fail to see that: Depending on the hypervisor's capabilities, the
>> two main functions could be much smaller (potentially there wouldn't
>> even be a need for the unlock hook in some cases),
> 
> What mechanism are you envisaging in that case?

A simple yield is better than not doing anything at all.

>> The list really juts is needed to not pointlessly tickle CPUs that
>> won't own the just released lock next anyway (or would own
>> it, but meanwhile went for another one where they also decided
>> to go into polling mode).
> 
> Did you measure that it was a particularly common case which was worth
> optimising for?

I didn't measure this particular case. But since the main problem
with ticket locks is when (host) CPUs are overcommitted, it
certainly is a bad idea to create even more load on the host than
there already is (the more that these are bursts).

Jan

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