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Message-ID: <20130408141400.GF17178@dhcp22.suse.cz>
Date: Mon, 8 Apr 2013 16:14:00 +0200
From: Michal Hocko <mhocko@...e.cz>
To: Li Zefan <lizefan@...wei.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Tejun Heo <tj@...nel.org>,
Glauber Costa <glommer@...allels.com>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
Johannes Weiner <hannes@...xchg.org>,
LKML <linux-kernel@...r.kernel.org>,
Cgroups <cgroups@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH 07/12] memcg: use css_get/put when charging/uncharging
kmem
On Mon 08-04-13 14:34:35, Li Zefan wrote:
> Use css_get/put instead of mem_cgroup_get/put.
>
> We can't do a simple replacement, because here mem_cgroup_put()
> is called during mem_cgroup_css_free(), while mem_cgroup_css_free()
> won't be called until css refcnt goes down to 0.
>
> Instead we increment css refcnt in mem_cgroup_css_offline(), and
> then check if there's still kmem charges. If not, css refcnt will
> be decremented immediately, otherwise the refcnt won't be decremented
> when kmem charges goes down to 0.
>
> v2:
> - added wmb() in kmem_cgroup_css_offline(), pointed out by Michal
> - revised comments as suggested by Michal
> - fixed to check if kmem is activated in kmem_cgroup_css_offline()
>
> Signed-off-by: Li Zefan <lizefan@...wei.com>
Acked-by: Michal Hocko <mhocko@...e.cz>
Thanks!
> ---
> mm/memcontrol.c | 66 +++++++++++++++++++++++++++++++++++----------------------
> 1 file changed, 41 insertions(+), 25 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index c308ea0..7be796c 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3003,8 +3003,16 @@ static void memcg_uncharge_kmem(struct mem_cgroup *memcg, u64 size)
> if (res_counter_uncharge(&memcg->kmem, size))
> return;
>
> + /*
> + * Releases a reference taken in kmem_cgroup_css_offline in case
> + * this last uncharge is racing with the offlining code or it is
> + * outliving the memcg existence.
> + *
> + * The memory barrier imposed by test&clear is paired with the
> + * explicit one in kmem_cgroup_css_offline.
> + */
> if (memcg_kmem_test_and_clear_dead(memcg))
> - mem_cgroup_put(memcg);
> + css_put(&memcg->css);
> }
>
> void memcg_cache_list_add(struct mem_cgroup *memcg, struct kmem_cache *cachep)
> @@ -5090,14 +5098,6 @@ static int memcg_update_kmem_limit(struct cgroup *cont, u64 val)
> * starts accounting before all call sites are patched
> */
> memcg_kmem_set_active(memcg);
> -
> - /*
> - * kmem charges can outlive the cgroup. In the case of slab
> - * pages, for instance, a page contain objects from various
> - * processes, so it is unfeasible to migrate them away. We
> - * need to reference count the memcg because of that.
> - */
> - mem_cgroup_get(memcg);
> } else
> ret = res_counter_set_limit(&memcg->kmem, val);
> out:
> @@ -5130,12 +5130,10 @@ static int memcg_propagate_kmem(struct mem_cgroup *memcg)
> goto out;
>
> /*
> - * destroy(), called if we fail, will issue static_key_slow_inc() and
> - * mem_cgroup_put() if kmem is enabled. We have to either call them
> - * unconditionally, or clear the KMEM_ACTIVE flag. I personally find
> - * this more consistent, since it always leads to the same destroy path
> + * __mem_cgroup_free() will issue static_key_slow_dec() because this
> + * memcg is active already. If the later initialization fails then the
> + * cgroup core triggers the cleanup so we do not have to do it here.
> */
> - mem_cgroup_get(memcg);
> static_key_slow_inc(&memcg_kmem_enabled_key);
>
> mutex_lock(&set_limit_mutex);
> @@ -5818,23 +5816,39 @@ static int memcg_init_kmem(struct mem_cgroup *memcg, struct cgroup_subsys *ss)
> return mem_cgroup_sockets_init(memcg, ss);
> };
>
> -static void kmem_cgroup_destroy(struct mem_cgroup *memcg)
> +static void kmem_cgroup_css_offline(struct mem_cgroup *memcg)
> {
> - mem_cgroup_sockets_destroy(memcg);
> + if (!memcg_kmem_is_active(memcg))
> + return;
>
> + /*
> + * kmem charges can outlive the cgroup. In the case of slab
> + * pages, for instance, a page contain objects from various
> + * processes. As we prevent from taking a reference for every
> + * such allocation we have to be careful when doing uncharge
> + * (see memcg_uncharge_kmem) and here during offlining.
> + *
> + * The idea is that that only the _last_ uncharge which sees
> + * the dead memcg will drop the last reference. An additional
> + * reference is taken here before the group is marked dead
> + * which is then paired with css_put during uncharge resp. here.
> + *
> + * Although this might sound strange as this path is called when
> + * the reference has already dropped down to 0 and shouldn't be
> + * incremented anymore (css_tryget would fail) we do not have
> + * other options because of the kmem allocations lifetime.
> + */
> + css_get(&memcg->css);
> +
> + /* see comment in memcg_uncharge_kmem() */
> + wmb();
> memcg_kmem_mark_dead(memcg);
>
> if (res_counter_read_u64(&memcg->kmem, RES_USAGE) != 0)
> return;
>
> - /*
> - * Charges already down to 0, undo mem_cgroup_get() done in the charge
> - * path here, being careful not to race with memcg_uncharge_kmem: it is
> - * possible that the charges went down to 0 between mark_dead and the
> - * res_counter read, so in that case, we don't need the put
> - */
> if (memcg_kmem_test_and_clear_dead(memcg))
> - mem_cgroup_put(memcg);
> + css_put(&memcg->css);
> }
> #else
> static int memcg_init_kmem(struct mem_cgroup *memcg, struct cgroup_subsys *ss)
> @@ -5842,7 +5856,7 @@ static int memcg_init_kmem(struct mem_cgroup *memcg, struct cgroup_subsys *ss)
> return 0;
> }
>
> -static void kmem_cgroup_destroy(struct mem_cgroup *memcg)
> +static void kmem_cgroup_css_offline(struct mem_cgroup *memcg)
> {
> }
> #endif
> @@ -6268,6 +6282,8 @@ static void mem_cgroup_css_offline(struct cgroup *cont)
> {
> struct mem_cgroup *memcg = mem_cgroup_from_cont(cont);
>
> + kmem_cgroup_css_offline(memcg);
> +
> mem_cgroup_invalidate_reclaim_iterators(memcg);
> mem_cgroup_reparent_charges(memcg);
> mem_cgroup_destroy_all_caches(memcg);
> @@ -6277,7 +6293,7 @@ static void mem_cgroup_css_free(struct cgroup *cont)
> {
> struct mem_cgroup *memcg = mem_cgroup_from_cont(cont);
>
> - kmem_cgroup_destroy(memcg);
> + mem_cgroup_sockets_destroy(memcg);
>
> mem_cgroup_put(memcg);
> }
> --
> 1.8.0.2
> --
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--
Michal Hocko
SUSE Labs
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