Subject: Re: [PATCH -v11][RFC] mutex: implement adaptive spinning
From: Peter Zijlstra <peterz@infradead.org>
To: Nick Piggin <npiggin@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
       Ingo Molnar <mingo@elte.hu>,
       "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
       Gregory Haskins <ghaskins@novell.com>, Matthew Wilcox <matthew@wil.cx>,
       Andi Kleen <andi@firstfloor.org>, 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>,
       Peter Morreale <pmorreale@novell.com>,
       Sven Dietrich <SDietrich@novell.com>,
       Dmitry Adamushko <dmitry.adamushko@gmail.com>,
       Johannes Weiner <hannes@cmpxchg.org>
In-Reply-To: <20090115004645.GB32044@wotan.suse.de>
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Date: Thu, 15 Jan 2009 08:44:03 +0100
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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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