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Message-ID: <529CDBAB.8060307@parallels.com>
Date: Mon, 2 Dec 2013 23:12:43 +0400
From: Vladimir Davydov <vdavydov@...allels.com>
To: Michal Hocko <mhocko@...e.cz>
CC: <linux-kernel@...r.kernel.org>, <cgroups@...r.kernel.org>,
<devel@...nvz.org>, Johannes Weiner <hannes@...xchg.org>,
Balbir Singh <bsingharora@...il.com>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
Glauber Costa <glommer@...il.com>
Subject: Re: [PATCH] memcg: remove KMEM_ACCOUNTED_ACTIVATED
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@...allels.com>
>> Cc: Johannes Weiner <hannes@...xchg.org>
>> Cc: Michal Hocko <mhocko@...e.cz>
>> Cc: Balbir Singh <bsingharora@...il.com>
>> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...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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