linux-kernel - Re: [PATCH v2 1/3] mutex: Make more scalable by doing less atomic operations

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Message-ID: <516C128C.3040302@redhat.com>
Date:	Mon, 15 Apr 2013 10:45:32 -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 1/3] mutex: Make more scalable by doing less atomic
 operations

On 04/15/2013 10:37 AM, Waiman Long wrote:
> In the __mutex_lock_common() function, an initial entry into
> the lock slow path will cause two atomic_xchg instructions to be
> issued. Together with the atomic decrement in the fast path, a total
> of three atomic read-modify-write instructions will be issued in
> rapid succession. This can cause a lot of cache bouncing when many
> tasks are trying to acquire the mutex at the same time.
>
> This patch will reduce the number of atomic_xchg instructions used by
> checking the counter value first before issuing the instruction. The
> atomic_read() function is just a simple memory read. The atomic_xchg()
> function, on the other hand, can be up to 2 order of magnitude or even
> more in cost when compared with atomic_read(). By using atomic_read()
> to check the value first before calling atomic_xchg(), we can avoid a
> lot of unnecessary cache coherency traffic. The only downside with this
> change is that a task on the slow path will have a tiny bit
> less chance of getting the mutex when competing with another task
> in the fast path.
>

> Signed-off-by: Waiman Long <Waiman.Long@...com>
> Reviewed-by: Davidlohr Bueso <davidlohr.bueso@...com>

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


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