linux-kernel - Re: [PATCH -v5][RFC]: mutex: implement adaptive spinning

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Message-ID: <alpine.LFD.2.00.0901071510390.3057@localhost.localdomain>
Date:	Wed, 7 Jan 2009 15:15:48 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Gregory Haskins <ghaskins@...ell.com>
cc:	Andi Kleen <andi@...stfloor.org>, Matthew Wilcox <matthew@....cx>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	paulmck@...ux.vnet.ibm.com, Ingo Molnar <mingo@...e.hu>,
	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 -v5][RFC]: mutex: implement adaptive spinning



On Wed, 7 Jan 2009, Gregory Haskins wrote:
>
> Can I ask a simple question in light of all this discussion? 
> 
> "Is get_task_struct() really that bad?"

Yes. It's an atomic access (two, in fact, since you need to release it 
too), which is a huge deal if we're talking about a timing-critical 
section of code.

And this is timing-critical, or we wouldn't even care - even in the 
contention case. Admittedly btrfs apparently makes it more so that it 
_should_ be, but Peter had some timings that happened with just regular 
create/unlink that showed a big difference.

So the whole and only point of spinning mutexes is to get rid of the 
scheduler overhead, but to also not replace it with some other thing ;)

			Linus
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