linux-kernel - Re: [PATCH -v8][RFC] mutex: implement adaptive spinning

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Date:	Wed, 14 Jan 2009 18:04:45 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Avi Kivity <avi@...hat.com>,
	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>
Subject: Re: [PATCH -v8][RFC] mutex: implement adaptive spinning

On Wed, Jan 14, 2009 at 05:46:39PM +0100, Peter Zijlstra wrote:
> On Mon, 2009-01-12 at 19:32 +0200, Avi Kivity wrote:
> > Peter Zijlstra wrote:
> > > Spinlocks can use 'pure' MCS locks.
> > >   
> > 
> > How about this, then.  In mutex_lock(), keep wait_lock locked and only 
> > release it when scheduling out.  Waiter spinning naturally follows.  If 
> > spinlocks are cache friendly (are they today?) 
> 
> (no they're not, Nick's ticket locks still spin on a shared cacheline
> IIRC -- the MCS locks mentioned could fix this)

It reminds me. I wrote a basic variation of MCS spinlocks a while back. And
converted dcache lock to use it, which showed large dbench improvements on
a big machine (of course for different reasons than the dbench improvements
in this threaed).

http://lkml.org/lkml/2008/8/28/24

Each "lock" object is sane in size because given set of spin-local queues may
only be used once per lock stack. But any spinlocks within a mutex acquisition
will always be at the bottom of such a stack anyway, by definition.

If you can use any code or concept for your code, that would be great.

 
> > we inherit that.  If 
> > there is no contention on the mutex, then we don't need to reacquire the 
> > wait_lock on mutex_unlock() (not that the atomic op is that expensive 
> > these days).
> 
> That might actually work, although we'd have to move the
> __mutex_slowpath_needs_to_unlock() branch outside wait_lock otherwise
> we'll deadlock :-)
> 
> It might be worth trying this if we get serious fairness issues with the
> current construct.
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