linux-kernel - Re: [PATCH 1/1] mlock: split the shmlock_user_lock spinlock into per user

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Message-ID: <1336073474.6509.2.camel@twins>
Date:	Thu, 03 May 2012 21:31:14 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	rajman mekaco <rajman.mekaco@...il.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Christoph Lameter <cl@...two.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/1] mlock: split the shmlock_user_lock spinlock into
 per user_struct spinlock

On Thu, 2012-05-03 at 23:04 +0530, rajman mekaco wrote:
> The user_shm_lock and user_shm_unlock functions use a single global
> spinlock for protecting the user->locked_shm.

Are you very sure its only protecting user state? This changelog doesn't
convince me you've gone through everything and found it good.

> This is an overhead for multiple CPUs calling this code even if they
> are having different user_struct.
> 
> Remove the global shmlock_user_lock and introduce and use a new
> spinlock inside of the user_struct structure. 

While I don't immediately see anything wrong with it, I doubt its
useful. What workload run with enough users that this makes a difference
one way or another?


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