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Date: Tue, 14 Feb 2012 17:43:54 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To: Greg Thelen <gthelen@...gle.com>
Cc: "linux-mm@...ck.org" <linux-mm@...ck.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"hannes@...xchg.org" <hannes@...xchg.org>,
Michal Hocko <mhocko@...e.cz>,
"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
Ying Han <yinghan@...gle.com>, Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH 4/6 v4] memcg: use new logic for page stat accounting
On Mon, 13 Feb 2012 23:22:22 -0800
Greg Thelen <gthelen@...gle.com> wrote:
> On Mon, Feb 13, 2012 at 7:14 PM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@...fujitsu.com> wrote:
> > From ad2905362ef58a44d96a325193ab384739418050 Mon Sep 17 00:00:00 2001
> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > Date: Thu, 2 Feb 2012 11:49:59 +0900
> > Subject: [PATCH 4/6] memcg: use new logic for page stat accounting.
> >
> > Now, page-stat-per-memcg is recorded into per page_cgroup flag by
> > duplicating page's status into the flag. The reason is that memcg
> > has a feature to move a page from a group to another group and we
> > have race between "move" and "page stat accounting",
> >
> > Under current logic, assume CPU-A and CPU-B. CPU-A does "move"
> > and CPU-B does "page stat accounting".
> >
> > When CPU-A goes 1st,
> >
> > CPU-A CPU-B
> > update "struct page" info.
> > move_lock_mem_cgroup(memcg)
> > see flags
>
> pc->flags?
>
yes.
> > copy page stat to new group
> > overwrite pc->mem_cgroup.
> > move_unlock_mem_cgroup(memcg)
> > move_lock_mem_cgroup(mem)
> > set pc->flags
> > update page stat accounting
> > move_unlock_mem_cgroup(mem)
> >
> > stat accounting is guarded by move_lock_mem_cgroup() and "move"
> > logic (CPU-A) doesn't see changes in "struct page" information.
> >
> > But it's costly to have the same information both in 'struct page' and
> > 'struct page_cgroup'. And, there is a potential problem.
> >
> > For example, assume we have PG_dirty accounting in memcg.
> > PG_..is a flag for struct page.
> > PCG_ is a flag for struct page_cgroup.
> > (This is just an example. The same problem can be found in any
> > kind of page stat accounting.)
> >
> > CPU-A CPU-B
> > TestSet PG_dirty
> > (delay) TestClear PG_dirty_
>
> PG_dirty
>
> > if (TestClear(PCG_dirty))
> > memcg->nr_dirty--
> > if (TestSet(PCG_dirty))
> > memcg->nr_dirty++
> >
>
> > @@ -141,6 +141,31 @@ static inline bool mem_cgroup_disabled(void)
> > return false;
> > }
> >
> > +void __mem_cgroup_begin_update_page_stat(struct page *page,
> > + bool *lock, unsigned long *flags);
> > +
> > +static inline void mem_cgroup_begin_update_page_stat(struct page *page,
> > + bool *lock, unsigned long *flags)
> > +{
> > + if (mem_cgroup_disabled())
> > + return;
> > + rcu_read_lock();
> > + *lock = false;
>
> This seems like a strange place to set *lock=false. I think it's
> clearer if __mem_cgroup_begin_update_page_stat() is the only routine
> that sets or clears *lock. But I do see that in patch 6/6 'memcg: fix
> performance of mem_cgroup_begin_update_page_stat()' this position is
> required.
>
Ah, yes. Hmm, it was better to move this to the body of function.
> > + return __mem_cgroup_begin_update_page_stat(page, lock, flags);
> > +}
>
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index ecf8856..30afea5 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -1877,32 +1877,54 @@ bool mem_cgroup_handle_oom(struct mem_cgroup *memcg, gfp_t mask)
> > * If there is, we take a lock.
> > */
> >
> > +void __mem_cgroup_begin_update_page_stat(struct page *page,
> > + bool *lock, unsigned long *flags)
> > +{
> > + struct mem_cgroup *memcg;
> > + struct page_cgroup *pc;
> > +
> > + pc = lookup_page_cgroup(page);
> > +again:
> > + memcg = pc->mem_cgroup;
> > + if (unlikely(!memcg || !PageCgroupUsed(pc)))
> > + return;
> > + if (!mem_cgroup_stealed(memcg))
> > + return;
> > +
> > + move_lock_mem_cgroup(memcg, flags);
> > + if (memcg != pc->mem_cgroup || !PageCgroupUsed(pc)) {
> > + move_unlock_mem_cgroup(memcg, flags);
> > + goto again;
> > + }
> > + *lock = true;
> > +}
> > +
> > +void __mem_cgroup_end_update_page_stat(struct page *page,
> > + bool *lock, unsigned long *flags)
>
> 'lock' looks like an unused parameter. If so, then remove it.
>
Ok.
> > +{
> > + struct page_cgroup *pc = lookup_page_cgroup(page);
> > +
> > + /*
> > + * It's guaranteed that pc->mem_cgroup never changes while
> > + * lock is held
>
> Please continue comment describing what provides this guarantee. I
> assume it is because rcu_read_lock() is held by
> mem_cgroup_begin_update_page_stat(). Maybe it's best to to just make
> small reference to the locking protocol description in
> mem_cgroup_start_move().
>
Ok, I will update this.
> > + */
> > + move_unlock_mem_cgroup(pc->mem_cgroup, flags);
> > +}
> > +
> > +
>
> I think it would be useful to add a small comment here declaring that
> all callers of this routine must be in a
> mem_cgroup_begin_update_page_stat(), mem_cgroup_end_update_page_stat()
> critical section to keep pc->mem_cgroup stable.
>
Sure, will do.
Thank you for review.
-Kame
> > void mem_cgroup_update_page_stat(struct page *page,
> > enum mem_cgroup_page_stat_item idx, int val)
> > {
>
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