Re: [RFC] [PATCH 7/7 v2] memcg: make mem_cgroup_begin_update_stat to use global pcpu.

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Fri, 20 Jan 2012 00:40:34 -0800
Greg Thelen <gthelen@google.com> wrote:

> On Fri, Jan 13, 2012 at 12:45 AM, KAMEZAWA Hiroyuki

> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 8b67ccf..4836e8d 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -89,7 +89,6 @@ enum mem_cgroup_stat_index {
> > A  A  A  A MEM_CGROUP_STAT_FILE_MAPPED, A /* # of pages charged as file rss */
> > A  A  A  A MEM_CGROUP_STAT_SWAPOUT, /* # of pages, swapped out */
> > A  A  A  A MEM_CGROUP_STAT_DATA, /* end of data requires synchronization */
> > - A  A  A  MEM_CGROUP_ON_MOVE, A  A  /* someone is moving account between groups */
> > A  A  A  A MEM_CGROUP_STAT_NSTATS,
> > A };
> >
> > @@ -279,6 +278,8 @@ struct mem_cgroup {
> > A  A  A  A  * mem_cgroup ? And what type of charges should we move ?
> > A  A  A  A  */
> > A  A  A  A unsigned long A  move_charge_at_immigrate;
> > + A  A  A  /* set when a page under this memcg may be moving to other memcg */
> > + A  A  A  atomic_t A  A  A  A account_moving;
> > A  A  A  A /*
> > A  A  A  A  * percpu counter.
> > A  A  A  A  */
> > @@ -1250,20 +1251,27 @@ int mem_cgroup_swappiness(struct mem_cgroup *memcg)
> > A  A  A  A return memcg->swappiness;
> > A }
> >
> > +/*
> > + * For quick check, for avoiding looking up memcg, system-wide
> > + * per-cpu check is provided.
> > + */
> > +DEFINE_PER_CPU(int, mem_cgroup_account_moving);
> 
> Why is this a per-cpu counter?  Can this be an single atomic_t
> instead, or does cpu hotplug require per-cpu state?  In the common
> case, when there is no move in progress, then the counter would be
> zero and clean in all cpu caches that need it.  When moving pages,
> mem_cgroup_start_move() would atomic_inc the counter.
> 

Ok, atomic_t will be simple.

Thanks,
-Kame

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