linux-kernel - Re: [PATCH -v11][RFC] mutex: implement adaptive spinning

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Date:	Thu, 15 Jan 2009 08:44:03 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	"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>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH -v11][RFC] mutex: implement adaptive spinning

On Thu, 2009-01-15 at 01:46 +0100, Nick Piggin wrote:

> Hmm, well this is rather a slow path, I would say. I'd prefer not to
> modify schedule in this way (if we just get scheduled back on after
> being switched away, the subsequent call to schedule is going to be
> cache hot and not do too much work).
> 
> preempt_enable_noresched maybe if you really care, would close up the
> window even more. But is it really worthwhile? We'd want to see numbers
> (when in doubt, keep it  simpler).

I initially did the preempt_enable_no_resched() thing and that showed
some improvement for PREEMPT=y kernels (lost the numbers though).

When I redid all the patches I tried closing that last hole by doing
that __schedule() thing, never realizing that schedule() would then get
extra overhead,.. d'0h.

I agree that that isn't worth it. I shall revert to
preempt_enable_no_resched() and try to get some new numbers.

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