Re: [PATCH 03/14] memcg: update memcg_has_children() to use css_next_child()

Michal Hocko
2014-05-12T15:18:09Z

On Fri 09-05-14 17:31:20, Tejun Heo wrote:
> Currently, memcg_has_children() and mem_cgroup_hierarchy_write()
> directly test cgroup->children for list emptiness. It's semantically
> correct in traditional hierarchies as it actually wants to test for
> any children dead or alive; however, cgroup->children is not a
> published field and scheduled to go away.
>
> This patch moves out .use_hierarchy test out of memcg_has_children()
> and updates it to use css_next_child() to test whether there exists
> any children. With .use_hierarchy test moved out, it can also be used
> by mem_cgroup_hierarchy_write().

I hope we will not grow more users of memcg_has_children but it would be
good to at least add a comment that this has to be checked by the
caller because we consider parent/children always when use_hierarchy is
used.

> A side note: As .use_hierarchy is going away, it doesn't really matter
> but I'm not sure about how it's used in __memcg_activate_kmem(). The
> condition tested by memcg_has_children() is mushy when seen from
> userland as its result is affected by dead csses which aren't visible
> from userland. I think the rule would be a lot clearer if we have a
> dedicated "freshly minted" flag which gets cleared when the first task
> is migrated into it or the first child is created and then gate
> activation with that.

This would work. KMEM_ACCOUNTED_ALLOWED in kmem_account_flags would be a
candidate.

> Signed-off-by: Tejun Heo <tj@kernel.org>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@suse.cz>

Acked-by: Michal Hocko <mhocko@suse.cz>

> ---
> mm/memcontrol.c | 26 ++++++++++++++++----------
> 1 file changed, 16 insertions(+), 10 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 036453a..eb6e1b5 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4834,18 +4834,23 @@ static void mem_cgroup_reparent_charges(struct mem_cgroup *memcg)
> } while (usage > 0);
> }
>
> +/* test whether @memcg has children, dead or alive */
> static inline bool memcg_has_children(struct mem_cgroup *memcg)
> {
> - lockdep_assert_held(&memcg_create_mutex);
> + bool ret;
> +
> /*
> - * The lock does not prevent addition or deletion to the list
> - * of children, but it prevents a new child from being
> - * initialized based on this parent in css_online(), so it's
> - * enough to decide whether hierarchically inherited
> - * attributes can still be changed or not.
> + * The lock does not prevent addition or deletion of children, but
> + * it prevents a new child from being initialized based on this
> + * parent in css_online(), so it's enough to decide whether
> + * hierarchically inherited attributes can still be changed or not.
> */
> - return memcg->use_hierarchy &&
> - !list_empty(&memcg->css.cgroup->children);
> + lockdep_assert_held(&memcg_create_mutex);
> +
> + rcu_read_lock();
> + ret = css_next_child(NULL, &memcg->css);
> + rcu_read_unlock();
> + return ret;
> }
>
> /*
> @@ -4921,7 +4926,7 @@ static int mem_cgroup_hierarchy_write(struct cgroup_subsys_state *css,
> */
> if ((!parent_memcg || !parent_memcg->use_hierarchy) &&
> (val == 1 || val == 0)) {
> - if (list_empty(&memcg->css.cgroup->children))
> + if (!memcg_has_children(memcg))
> memcg->use_hierarchy = val;
> else
> retval = -EBUSY;
> @@ -5038,7 +5043,8 @@ static int __memcg_activate_kmem(struct mem_cgroup *memcg,
> * of course permitted.
> */
> mutex_lock(&memcg_create_mutex);
> - if (cgroup_has_tasks(memcg->css.cgroup) || memcg_has_children(memcg))
> + if (cgroup_has_tasks(memcg->css.cgroup) ||
> + (memcg->use_hierarchy && memcg_has_children(memcg)))
> err = -EBUSY;
> mutex_unlock(&memcg_create_mutex);
> if (err)
> --
> 1.9.0
>

--
Michal Hocko
SUSE Labs