Re: [PATCH] memcg: remove KMEM_ACCOUNTED_ACTIVATED

[Vladimir Davydov <vdavydov@parallels.com>]

On 12/02/2013 10:26 PM, Glauber Costa wrote:
> On Mon, Dec 2, 2013 at 10:15 PM, Michal Hocko <mhocko@suse.cz> 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.
>>
>> Or am I missing something?
> You are correct. This flag is there due to the way we are using static branches.
> The patching of one call site is atomic, but the patching of all of
> them are not.
> Therefore we need to use a two-flag scheme to guarantee that in the first time
> we turn the static branches on, there will be a clear point after
> which we're going
> to start accounting.

Hi, Glauber

Sorry, but I don't understand why we need two flags. Isn't checking the 
flag set after all call sites have been patched (I mean 
KMEM_ACCOUNTED_ACTIVE) not enough?