Re: [PATCH] memcg: remove KMEM_ACCOUNTED_ACTIVATED

[Michal Hocko]

[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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