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SubjectRe: [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@jp.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@google.com>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Much better!

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


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