Re: [PATCH 2/6] memcg: simplify move_account() check.

On Fri, Feb 17, 2012 at 06:26:12PM +0900, KAMEZAWA Hiroyuki wrote:

quoted

From 3b6620772d7fd7b2126d5253eafb6afaf4ed6e34 Mon Sep 17 00:00:00 2001

From: KAMEZAWA Hiroyuki <redacted>
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 <redacted>
Signed-off-by: KAMEZAWA Hiroyuki <redacted>

Much better!

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

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