Re: [RFC][PATCH 3/7] memcg: use css_get/put when charging/uncharging kmem

Michal Hocko
2013-04-04T09:43:33Z

On Wed 03-04-13 17:12:36, 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, otherwise the refcnt will be decremented when
> kmem charges goes down to 0.

Yes the patch makes sense and I actually like it. We just have to be
sure to document the reference counting as much as possible (see
bellow). I also thing that we need a memory barrier in the offline code.

> Signed-off-by: Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>
> ---
> mm/memcontrol.c | 49 ++++++++++++++++++++++++++-----------------------
> 1 file changed, 26 insertions(+), 23 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index dafacb8..877551d 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3004,7 +3004,7 @@ static void memcg_uncharge_kmem(struct mem_cgroup *memcg, u64 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 pair 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)
> @@ -5089,14 +5089,6 @@ 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:
> @@ -5129,12 +5121,11 @@ 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
> + * destroy(), called if we fail, will issue static_key_slow_dec() 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
> */

Can we make this comment more clear while you are changing it, please? E.g.
/*
* __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);
> @@ -5823,23 +5814,33 @@ 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);
> + /*
> + * 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.
> + */

I would prefer if we could merge all three comments in this function
into a single one. What about something like the following?
/*
* 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);

I think that you need a write memory barrier here because css_get
nor memcg_kmem_mark_dead implies it. memcg_uncharge_kmem uses
memcg_kmem_test_and_clear_dead which imply a full memory barrier but it
should see the elevated reference count. No?

> + /*
> + * We need to call css_get() first, because memcg_uncharge_kmem()
> + * will call css_put() if it sees the memcg is dead.
> + */
> 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
> + * Charges already down to 0, undo css_get() done previosly,, 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)
> @@ -5847,7 +5848,7 @@ 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
> @@ -6274,6 +6275,8 @@ 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);
> @@ -6283,7 +6286,7 @@ 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

--
Michal Hocko
SUSE Labs