linux-kernel - Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

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Date:	Fri, 09 Jan 2009 16:06:54 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...e.hu>, paulmck@...ux.vnet.ibm.com,
	Gregory Haskins <ghaskins@...ell.com>,
	Matthew Wilcox <matthew@....cx>,
	Andi Kleen <andi@...stfloor.org>,
	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 -v7][RFC]: mutex: implement adaptive spinning

On Fri, 2009-01-09 at 11:47 +0100, Peter Zijlstra wrote:

> > So I think the bug is still there, we just hid it better by breaking out 
> > of the loop with that "if (need_resched())" always eventually triggering. 
> > And it would be ok if it really is guaranteed to _eventually_ trigger, and 
> > I guess with timeslices it eventually always will, but I suspect we could 
> > have some serious latency spikes.
> 
> Yes, the owner getting preempted after acquiring the lock, but before
> setting the owner can give some nasties :-(
> 
> I initially did that preempt_disable/enable around the fast path, but I
> agree that slowing down the fast path is unwelcome.
> 
> Alternatively we could go back to block on !owner, with the added
> complexity of not breaking out of the spin on lock->owner != owner
> when !lock->owner, so that the premature owner clearing of the unlock
> fast path will not force a schedule right before we get a chance to
> acquire the lock.
> 
> Let me do that..

Ok a few observations..

Adding that need_resched() in the outer loop utterly destroys the
performance gain for PREEMPT=y. Voluntary preemption is mostly good, but
somewhat unstable results.

Adding that blocking on !owner utterly destroys everything.

Going to look into where that extra preemption comes from.
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