linux-kernel - Re: [PATCH] memcg: remove unneeded preempt

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Date:	Thu, 18 Aug 2011 11:38:00 +0200
From:	Johannes Weiner <jweiner@...hat.com>
To:	Greg Thelen <gthelen@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Balbir Singh <bsingharora@...il.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Subject: Re: [PATCH] memcg: remove unneeded preempt_disable

On Wed, Aug 17, 2011 at 11:50:53PM -0700, Greg Thelen wrote:
> Both mem_cgroup_charge_statistics() and mem_cgroup_move_account() were
> unnecessarily disabling preemption when adjusting per-cpu counters:
>     preempt_disable()
>     __this_cpu_xxx()
>     __this_cpu_yyy()
>     preempt_enable()
> 
> This change does not disable preemption and thus CPU switch is possible
> within these routines.  This does not cause a problem because the total
> of all cpu counters is summed when reporting stats.  Now both
> mem_cgroup_charge_statistics() and mem_cgroup_move_account() look like:
>     this_cpu_xxx()
>     this_cpu_yyy()

Note that on non-x86, these operations themselves actually disable and
reenable preemption each time, so you trade a pair of add and sub on
x86

-	preempt_disable()
	__this_cpu_xxx()
	__this_cpu_yyy()
-	preempt_enable()

with

	preempt_disable()
	__this_cpu_xxx()
+	preempt_enable()
+	preempt_disable()
	__this_cpu_yyy()
	preempt_enable()

everywhere else.
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