linux-kernel - Re: [PATCH v2] mutex: do not unnecessarily deal with waiters

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Date:	Fri, 28 Jun 2013 16:53:15 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Davidlohr Bueso <davidlohr.bueso@...com>
CC:	Maarten Lankhorst <maarten.lankhorst@...onical.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] mutex: do not unnecessarily deal with waiters

On 06/28/2013 04:13 PM, Davidlohr Bueso wrote:
> From: Davidlohr Bueso <davidlohr.bueso@...com>
>
> Upon entering the slowpath, we immediately attempt to acquire the lock
> by checking if it is already unlocked. If we are lucky enough that this
> is the case, then we don't need to deal with any waiter related logic.
>
> Furthermore any checks for an empty wait_list are unnecessary as we
> already know that count is non-negative and hence no one is waiting for
> the lock.
>
> Move the count check and xchg calls to be done before any waiters are
> setup - including waiter debugging. Upon failure to acquire the lock,
> the xchg sets the counter to 0, instead of -1 as it was originally.
> This can be done here since we set it back to -1 right at the beginning
> of the loop so other waiters are woken up when the lock is released.
>
> When tested on a 8-socket (80 core) system against a vanilla 3.10-rc1
> kernel, this patch provides some small performance benefits (+2-6%).
> While it could be considered in the noise level, the average percentages
> were stable across multiple runs and no performance regressions were seen.
> Two big winners, for small amounts of users (10-100), were the short and
> compute workloads had a +19.36% and +%15.76% in jobs per minute.
>
> Also change some break statements to 'goto slowpath', which IMO makes a
> little more intuitive to read.
>
> Signed-off-by: Davidlohr Bueso <davidlohr.bueso@...com>

Acked-by: Rik van Riel <riel@...hat.com>


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