linux-kernel - Re: [PATCH 2/6] memcg: simplify move

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Message-ID: <20120220085521.GB1677@cmpxchg.org>
Date:	Mon, 20 Feb 2012 09:55:21 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>, Hugh Dickins <hughd@...gle.com>,
	Greg Thelen <gthelen@...gle.com>, Ying Han <yinghan@...gle.com>
Subject: Re: [PATCH 2/6] memcg: simplify move_account() check.

On Fri, Feb 17, 2012 at 06:26:12PM +0900, KAMEZAWA Hiroyuki wrote:
> >From 3b6620772d7fd7b2126d5253eafb6afaf4ed6e34 Mon Sep 17 00:00:00 2001
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> Date: Thu, 2 Feb 2012 10:02:39 +0900
> Subject: [PATCH 2/6] memcg: simplify move_account() check.
> 
> In memcg, for avoiding take-lock-irq-off at accessing page_cgroup,
> a logic, flag + rcu_read_lock(), is used. This works as following
> 
>      CPU-A                     CPU-B
>                              rcu_read_lock()
>     set flag
>                              if(flag is set)
>                                    take heavy lock
>                              do job.
>     synchronize_rcu()        rcu_read_unlock()
>     take heavy lock.
> 
> In recent discussion, it's argued that using per-cpu value for this
> flag just complicates the code because 'set flag' is very rare.
> 
> This patch changes 'flag' implementation from percpu to atomic_t.
> This will be much simpler.
> 
> Changelog v5.
>  - removed redundant ().
>  - updated patch description.
> 
> Changelog: v4
>  - fixed many typos.
>  - fixed return value to be bool
>  - add comments.
> Changelog: v3
>  - this is a new patch since v3.
> 
> Acked-by: Greg Thelen <gthelen@...gle.com>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

Much better!

Acked-by: Johannes Weiner <hannes@...xchg.org>
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