linux-kernel - Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

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Date:	Fri, 09 Jan 2009 17:03:51 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Chris Mason <chris.mason@...cle.com>,
	Ingo Molnar <mingo@...e.hu>, paulmck@...ux.vnet.ibm.com,
	Gregory Haskins <ghaskins@...ell.com>,
	Matthew Wilcox <matthew@....cx>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nick Piggin <npiggin@...e.de>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>
Subject: Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

On Fri, 2009-01-09 at 10:59 -0500, Steven Rostedt wrote:

> > 
> > Adding that blocking on !owner utterly destroys everything.
> 
> I was going to warn you about that ;-)
> 
> Without the check for !owner, you are almost guaranteed to go to sleep 
> every time. Here's why:
> 
> You are spinning and thus have a hot cache on that CPU.
> 
> The owner goes to unlock but will be in a cold cache. It sets lock->owner 
> to NULL, but is still in cold cache so it is a bit slower.
> 
> Once the spinner sees the NULL, it shoots out of the spin but sees the 
> lock is still not available then goes to sleep. All before the owner could 
> release it. This could probably happen at every contention. Thus, you lose 
> the benefit of spinning. You probably make things worse because you add a 
> spin before every sleep.

Which is why I changed the inner loop to:

  l_owner = ACCESS_ONCE(lock->owner)
  if (l_owner && l_owner != owner)
    break

So that that would continue spinning.
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