[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.
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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--
Michal Hocko
SUSE Labs
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