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

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Message-Id: <1231428605.11687.466.camel@twins>
Date:	Thu, 08 Jan 2009 16:30:05 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Gregory Haskins <ghaskins@...ell.com>,
	Matthew Wilcox <matthew@....cx>,
	Andi Kleen <andi@...stfloor.org>,
	Chris Mason <chris.mason@...cle.com>,
	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 Thu, 2009-01-08 at 10:28 -0500, Steven Rostedt wrote:
> On Thu, 8 Jan 2009, Peter Zijlstra wrote:

> > in the unlikely case we schedule(), that seems expensive enough to want
> > to make the spin case ever so slightly faster.
> 
> OK, that makes sense, but I would comment that. Otherwise, it just looks 
> like another misuse of the unlikely annotation.

OK, sensible enough.

> > > Should we need to do a "get_cpu" or something? Couldn't the CPU disappear 
> > > between these two calls. Or does it do a stop-machine and the preempt 
> > > disable will protect us?
> > 
> > Did you miss the preempt_disable() a bit up?
> 
> No, let me rephrase it better. Does the preempt_disable protect against
> another CPU from going off line? Does taking a CPU off line do a 
> stop_machine?

Yes and yes.
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