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

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Date:	Wed, 14 Jan 2009 17:46:39 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Avi Kivity <avi@...hat.com>
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>,
	Nick Piggin <npiggin@...e.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 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)

> 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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