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

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Message-Id: <20110906190424.ad5cc647.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Tue, 6 Sep 2011 19:04:24 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Johannes Weiner <jweiner@...hat.com>
Cc:	Greg Thelen <gthelen@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Balbir Singh <bsingharora@...il.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Subject: Re: [PATCH] memcg: remove unneeded preempt_disable

On Tue, 6 Sep 2011 11:58:52 +0200
Johannes Weiner <jweiner@...hat.com> wrote:

> 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()
> > 
> > Reported-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > Signed-off-by: Greg Thelen <gthelen@...gle.com>
> 
> I just noticed that both cases have preemption disabled anyway because
> of the page_cgroup bit spinlock.
> 
> So removing the preempt_disable() is fine but we can even keep the
> non-atomic __this_cpu operations.
> 
> Something like this instead?
> 
> ---
> From: Johannes Weiner <jweiner@...hat.com>
> Subject: mm: memcg: remove needless recursive preemption disabling
> 
> Callsites of mem_cgroup_charge_statistics() hold the page_cgroup bit
> spinlock, which implies disabled preemption.
> 
> The same goes for the explicit preemption disabling to account mapped
> file pages in mem_cgroup_move_account().
> 
> The explicit disabling of preemption in both cases is redundant.
> 
> Signed-off-by: Johannes Weiner <jweiner@...hat.com>

Could you add comments as
"This operation is called under bit spin lock !" ?

Nice catch.

Acked-by: KAMEZAWA Hiroyuki <kamezawa.hioryu@...fujitsu.com>

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