linux-kernel - Re: [PATCH 5/8] locking, mutex: Cancelable MCS lock for adaptive spinning

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Date:	Tue, 11 Feb 2014 10:18:05 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Low <jason.low2@...com>
Cc:	linux-kernel@...r.kernel.org, Waiman Long <waiman.long@...com>,
	mingo@...nel.org, paulmck@...ux.vnet.ibm.com,
	torvalds@...ux-foundation.org, tglx@...utronix.de, riel@...hat.com,
	akpm@...ux-foundation.org, davidlohr@...com, hpa@...or.com,
	andi@...stfloor.org, aswin@...com, scott.norton@...com,
	chegu_vinod@...com
Subject: Re: [PATCH 5/8] locking, mutex: Cancelable MCS lock for adaptive
 spinning

On Mon, Feb 10, 2014 at 02:04:22PM -0800, Jason Low wrote:
> On Mon, 2014-02-10 at 22:32 +0100, Peter Zijlstra wrote:
> > Is adding that really much faster than the relatively straight path
> > oqs_wait_next() would walk to bit the same exit?
> > 
> > The only reason I pulled out the above cmpxchg() is because its the
> > uncontended fast path, which seems like a special enough case.
> 
> So it would avoid 2 extra checks (*lock == node) and (node->next) in the
> oqs_wait_next() path, which aren't necessary when node->next != NULL.
> 
> And I think node->next != NULL can be considered a special enough case
> after the cmpxchg() fails because in the contended case, we're expecting
> the node->next to be pointing at something. The only times node->next is
> NULL after cmpxchg() fails are during a very small race window with the
> osq_lock(), and when the next node is unqueuing due to need_resched,
> which is also a very small window.

True all; now if only we had a useful benchmark so we could test if it
makes a difference or not :-)
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