[PATCH v3 5/9] memcg: use css_get/put when charging/uncharging kmem

Li Zefan
2013-06-13T09:19:31Z

Sorry for updating the patchset so late.

I've made some changes for the memory barrier thing, and I agree with
Michal that there can be improvement but can be a separate patch.

If this version is ok for everyone, I'll send the whole patchset out
to Andrew.

=========================

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.

v3:
- changed wmb() to smp_smb(), and moved it to memcg_kmem_mark_dead(),
and added comment.

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@huawei.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
mm/memcontrol.c | 70 ++++++++++++++++++++++++++++++++++++---------------------
1 file changed, 45 insertions(+), 25 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 466c595..76dcd0e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -416,6 +416,11 @@ static void memcg_kmem_clear_activated(struct mem_cgroup *memcg)

static void memcg_kmem_mark_dead(struct mem_cgroup *memcg)
{
+ /*
+ * We need to call css_get() first, because memcg_uncharge_kmem()
+ * will call css_put() if it sees the memcg is dead.
+ */
+ smb_wmb();
if (test_bit(KMEM_ACCOUNTED_ACTIVE, &memcg->kmem_account_flags))
set_bit(KMEM_ACCOUNTED_DEAD, &memcg->kmem_account_flags);
}
@@ -3060,8 +3065,16 @@ 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 memcg_kmem_mark_dead().
+ */
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)
@@ -5165,14 +5178,6 @@ 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:
@@ -5205,12 +5210,10 @@ 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);
@@ -5893,23 +5896,38 @@ 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 from
+ * css_offline() when the referencemight have 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);

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)
@@ -5917,7 +5935,7 @@ 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
@@ -6350,6 +6368,8 @@ 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);
@@ -6359,7 +6379,7 @@ 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