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Message-ID: <CAHH2K0a45xCTFz5qD-M_wX4DqsyfOZeL_G2JSs5NdHp1ZLHT_g@mail.gmail.com>
Date: Mon, 13 Feb 2012 23:22:22 -0800
From: Greg Thelen <gthelen@...gle.com>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.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, 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?
> 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.
> + 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.
> +{
> + 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().
> + */
> + 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.
> void mem_cgroup_update_page_stat(struct page *page,
> enum mem_cgroup_page_stat_item idx, int val)
> {
--
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