Date: Wed, 7 Jan 2009 15:15:48 -0800 (PST)
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Gregory Haskins <ghaskins@novell.com>
cc: Andi Kleen <andi@firstfloor.org>, Matthew Wilcox <matthew@wil.cx>,
       Steven Rostedt <rostedt@goodmis.org>,
       Peter Zijlstra <peterz@infradead.org>, paulmck@linux.vnet.ibm.com,
       Ingo Molnar <mingo@elte.hu>, Chris Mason <chris.mason@oracle.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       linux-fsdevel <linux-fsdevel@vger.kernel.org>,
       linux-btrfs <linux-btrfs@vger.kernel.org>,
       Thomas Gleixner <tglx@linutronix.de>, Nick Piggin <npiggin@suse.de>,
       Peter Morreale <pmorreale@novell.com>,
       Sven Dietrich <SDietrich@novell.com>
Subject: Re: [PATCH -v5][RFC]: mutex: implement adaptive spinning
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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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