linux-kernel - Re: [PATCH v2 2/3] mutex: Queue mutex spinners with MCS lock to reduce cacheline contention

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Date:	Mon, 15 Apr 2013 12:27:12 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Waiman Long <Waiman.Long@...com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	David Howells <dhowells@...hat.com>,
	Dave Jones <davej@...hat.com>,
	Clark Williams <williams@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, x86@...nel.org,
	linux-arch@...r.kernel.org,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH v2 2/3] mutex: Queue mutex spinners with MCS lock to reduce
 cacheline contention

On 04/15/2013 10:37 AM, Waiman Long wrote:
> The current mutex spinning code (with MUTEX_SPIN_ON_OWNER option turned
> on) allow multiple tasks to spin on a single mutex concurrently. A
> potential problem with the current approach is that when the mutex
> becomes available, all the spinning tasks will try to acquire the
> mutex more or less simultaneously. As a result, there will be a lot of
> cacheline bouncing especially on systems with a large number of CPUs.
>
> This patch tries to reduce this kind of contention by putting the
> mutex spinners into a queue so that only the first one in the queue
> will try to acquire the mutex. This will reduce contention and allow
> all the tasks to move forward faster.
>
> The queuing of mutex spinners is done using an MCS lock based
> implementation which will further reduce contention on the mutex
> cacheline than a similar ticket spinlock based implementation. This
> patch will add a new field into the mutex data structure for holding
> the MCS lock. This expands the mutex size by 8 bytes for 64-bit system
> and 4 bytes for 32-bit system. This overhead will be avoid if the
> MUTEX_SPIN_ON_OWNER option is turned off.

> Signed-off-by: Waiman Long <Waiman.Long@...com>

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

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