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

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Date:	Tue, 6 Sep 2011 11:04:16 -0700
From:	Greg Thelen <gthelen@...gle.com>
To:	Johannes Weiner <jweiner@...hat.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 Tue, Sep 6, 2011 at 2:58 AM, 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>

Looks good, thanks.

Reviewed-by: Greg Thelen <gthelen@...gle.com>

> ---
>  mm/memcontrol.c |    6 ------
>  1 file changed, 6 deletions(-)
>
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -618,8 +618,6 @@ static unsigned long mem_cgroup_read_eve
>  static void mem_cgroup_charge_statistics(struct mem_cgroup *mem,
>                                         bool file, int nr_pages)
>  {
> -       preempt_disable();
> -
>        if (file)
>                __this_cpu_add(mem->stat->count[MEM_CGROUP_STAT_CACHE], nr_pages);
>        else
> @@ -634,8 +632,6 @@ static void mem_cgroup_charge_statistics
>        }
>
>        __this_cpu_add(mem->stat->events[MEM_CGROUP_EVENTS_COUNT], nr_pages);
> -
> -       preempt_enable();
>  }
>
>  unsigned long
> @@ -2582,10 +2578,8 @@ static int mem_cgroup_move_account(struc
>
>        if (PageCgroupFileMapped(pc)) {
>                /* Update mapped_file data for mem_cgroup */
> -               preempt_disable();
>                __this_cpu_dec(from->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
>                __this_cpu_inc(to->stat->count[MEM_CGROUP_STAT_FILE_MAPPED]);
> -               preempt_enable();
>        }
>        mem_cgroup_charge_statistics(from, PageCgroupCache(pc), -nr_pages);
>        if (uncharge)
>
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