Re: [PATCH] memcg: remove KMEM_ACCOUNTED_ACTIVATED

[Glauber Costa <glommer@gmail.com>]

On Mon, Dec 2, 2013 at 10:51 PM, Michal Hocko <mhocko@suse.cz> wrote:
> On Mon 02-12-13 22:26:48, 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.
>
> So http://lkml.org/lkml/2013/11/27/314 is correct then, right?

It looks correct.