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

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Date:	Wed, 07 Jan 2009 22:28:39 -0500
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...e.hu>,
	Andi Kleen <andi@...stfloor.org>,
	Matthew Wilcox <matthew@....cx>,
	Peter Zijlstra <peterz@...radead.org>,
	paulmck@...ux.vnet.ibm.com, 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

[resend: restore CC list]

>>> Linus Torvalds <torvalds@...ux-foundation.org> 01/07/09 6:33 PM >>>


>On Wed, 7 Jan 2009, Steven Rostedt wrote:
>>
>> What would be interesting is various benchmarks against all three.
>>
>> 1) no mutex spinning.
>> 2) get_task_struct() implementation.
>> 3) spin_or_sched implementation.
>
>One of the issues is that I cannot convince myself that (2) is even
>necessarily correct. At least not without having all cases happen inder
>the mutex spinlock - which they don't. Even with the original patch, the
>uncontended cases set and cleared the owner field outside the lock.

Yeah, you are right (see my last post).  Without the cmpxchg trickery
(*) on the owner field (like we have in -rt) with the pending bits, (2)
cannot work.

(*) For what its worth, I am impressed with the brilliance of the
cmgxchg fast path stuff in the rtmutex code.  Whoever came up with that,
my hat is off to you.

-Greg
 


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