Re: [PATCH] memcg: remove KMEM_ACCOUNTED_ACTIVATED

Vladimir Davydov Mon, 02 Dec 2013 13:20:03 -0800

On 12/02/2013 10:15 PM, Michal Hocko wrote:

[CCing Glauber - please do so in other posts for kmem related changes]


On Mon 02-12-13 17:08:13, Vladimir Davydov wrote:

The KMEM_ACCOUNTED_ACTIVATED was introduced by commit a8964b9b ("memcg:
use static branches when code not in use") in order to guarantee that
static_key_slow_inc(&memcg_kmem_enabled_key) will be called only once
for each memory cgroup when its kmem limit is set. The point is that at
that time the memcg_update_kmem_limit() function's workflow looked like
this:

        bool must_inc_static_branch = false;

        cgroup_lock();
        mutex_lock(&set_limit_mutex);
        if (!memcg->kmem_account_flags && val != RESOURCE_MAX) {
                /* The kmem limit is set for the first time */
                ret = res_counter_set_limit(&memcg->kmem, val);

                memcg_kmem_set_activated(memcg);
                must_inc_static_branch = true;
        } else
                ret = res_counter_set_limit(&memcg->kmem, val);
        mutex_unlock(&set_limit_mutex);
        cgroup_unlock();

        if (must_inc_static_branch) {
                /* We can't do this under cgroup_lock */
                static_key_slow_inc(&memcg_kmem_enabled_key);
                memcg_kmem_set_active(memcg);
        }

Today, we don't use cgroup_lock in memcg_update_kmem_limit(), and
static_key_slow_inc() is called under the set_limit_mutex, but the
leftover from the above-mentioned commit is still here. Let's remove it.

OK, so I have looked there again and 692e89abd154b (memcg: increment
static branch right after limit set) which went in after cgroup_mutex
has been removed. It came along with the following comment.
                 /*
                  * setting the active bit after the inc will guarantee no one
                  * starts accounting before all call sites are patched
                  */

This suggests that the flag is needed after all because we have
to be sure that _all_ the places have to be patched. AFAIU
memcg_kmem_newpage_charge might see the static key already patched so
it would do a charge but memcg_kmem_commit_charge would still see it
unpatched and so the charge won't be committed.

Hmm, but we have the KMEM_ACCOUNTED_ACTIVE flag, which is set *after*static_key_slow_inc() in memcg_update_kmem_limit(), and__memcg_kmem_newpage_charge() checks it before charging(memcg_can_account_kmem()). So the situation you described is impossible?


Or am I missing something?

Signed-off-by: Vladimir Davydov <vdavy...@parallels.com>
Cc: Johannes Weiner <han...@cmpxchg.org>
Cc: Michal Hocko <mho...@suse.cz>
Cc: Balbir Singh <bsinghar...@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hir...@jp.fujitsu.com>
---
  mm/memcontrol.c |   26 +-------------------------
  1 file changed, 1 insertion(+), 25 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f1a0ae6..f78661f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -344,14 +344,9 @@ static size_t memcg_size(void)
  /* internal only representation about the status of kmem accounting. */
  enum {
        KMEM_ACCOUNTED_ACTIVE = 0, /* accounted by this cgroup itself */
-       KMEM_ACCOUNTED_ACTIVATED, /* static key enabled. */
        KMEM_ACCOUNTED_DEAD, /* dead memcg with pending kmem charges */
  };

-/* We account when limit is on, but only after call sites are patched */

-#define KMEM_ACCOUNTED_MASK \
-               ((1 << KMEM_ACCOUNTED_ACTIVE) | (1 << KMEM_ACCOUNTED_ACTIVATED))
-
  #ifdef CONFIG_MEMCG_KMEM
  static inline void memcg_kmem_set_active(struct mem_cgroup *memcg)
  {
@@ -363,16 +358,6 @@ static bool memcg_kmem_is_active(struct mem_cgroup *memcg)
        return test_bit(KMEM_ACCOUNTED_ACTIVE, &memcg->kmem_account_flags);
  }

-static void memcg_kmem_set_activated(struct mem_cgroup *memcg)

-{
-       set_bit(KMEM_ACCOUNTED_ACTIVATED, &memcg->kmem_account_flags);
-}
-
-static void memcg_kmem_clear_activated(struct mem_cgroup *memcg)
-{
-       clear_bit(KMEM_ACCOUNTED_ACTIVATED, &memcg->kmem_account_flags);
-}
-
  static void memcg_kmem_mark_dead(struct mem_cgroup *memcg)
  {
        /*
@@ -2956,7 +2941,7 @@ static DEFINE_MUTEX(set_limit_mutex);
  static inline bool memcg_can_account_kmem(struct mem_cgroup *memcg)
  {
        return !mem_cgroup_disabled() && !mem_cgroup_is_root(memcg) &&
-               (memcg->kmem_account_flags & KMEM_ACCOUNTED_MASK);
+               memcg_kmem_is_active(memcg);
  }

/*

@@ -3091,19 +3076,10 @@ int memcg_update_cache_sizes(struct mem_cgroup *memcg)
                                0, MEMCG_CACHES_MAX_SIZE, GFP_KERNEL);
        if (num < 0)
                return num;
-       /*
-        * After this point, kmem_accounted (that we test atomically in
-        * the beginning of this conditional), is no longer 0. This
-        * guarantees only one process will set the following boolean
-        * to true. We don't need test_and_set because we're protected
-        * by the set_limit_mutex anyway.
-        */
-       memcg_kmem_set_activated(memcg);

ret = memcg_update_all_caches(num+1);

        if (ret) {
                ida_simple_remove(&kmem_limited_groups, num);
-               memcg_kmem_clear_activated(memcg);
                return ret;
        }

--

1.7.10.4

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Re: [PATCH] memcg: remove KMEM_ACCOUNTED_ACTIVATED

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