Re: [PATCH] memcg: remove KMEM_ACCOUNTED_ACTIVATED
From: Vladimir Davydov
Date: Mon Dec 02 2013 - 16:18:20 EST
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 <vdavydov@xxxxxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxx>
Cc: Balbir Singh <bsingharora@xxxxxxxxx>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
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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