On Mon 21-01-13 15:13:30, Glauber Costa wrote:
> Currently, we use cgroups' provided list of children to verify if it is
> safe to proceed with any value change that is dependent on the cgroup
> being empty.
>
> This is less than ideal, because it enforces a dependency over cgroup
> core that we would be better off without. The solution proposed here is
> to iterate over the child cgroups and if any is found that is already
> online, we bounce and return: we don't really care how many children we
> have, only if we have any.
>
> This is also made to be hierarchy aware. IOW, cgroups with hierarchy
> disabled, while they still exist, will be considered for the purpose of
> this interface as having no children.
>
> Signed-off-by: Glauber Costa <glommer@xxxxxxxxxxxxx>
OK, as I said I can live with this.
Acked-by: Michal Hocko <mhocko@xxxxxxx>
But
> ---
> mm/memcontrol.c | 34 +++++++++++++++++++++++++++++-----
> 1 file changed, 29 insertions(+), 5 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 6c72204..6d3ad21 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4716,6 +4716,32 @@ static void mem_cgroup_reparent_charges(struct mem_cgroup *memcg)
> }
>
> /*
> + * this mainly exists for tests during set of use_hierarchy. Since this is
> + * the very setting we are changing, the current hierarchy value is meaningless
> + */
> +static inline bool __memcg_has_children(struct mem_cgroup *memcg)
> +{
> + struct cgroup *pos;
> +
> + /* bounce at first found */
> + cgroup_for_each_child(pos, memcg->css.cgroup)
> + return true;
> + return false;
> +}
This needs rcu_read_{un}lock.
> +
> +/*
> + * must be called with cgroup_lock held, unless the cgroup is guaranteed to be
> + * already dead (like in mem_cgroup_force_empty, for instance). This is
> + * different than mem_cgroup_count_children, in the sense that we don't really
> + * care how many children we have, we only need to know if we have any. It is
> + * also count any memcg without hierarchy as infertile for that matter.
> + */
> +static inline bool memcg_has_children(struct mem_cgroup *memcg)
> +{
> + return memcg->use_hierarchy && __memcg_has_children(memcg);
> +}
> +
> +/*
> * Reclaims as many pages from the given memcg as possible and moves
> * the rest to the parent.
> *
> @@ -4800,7 +4826,7 @@ static int mem_cgroup_hierarchy_write(struct cgroup *cont, struct cftype *cft,
> */
> if ((!parent_memcg || !parent_memcg->use_hierarchy) &&
> (val == 1 || val == 0)) {
> - if (list_empty(&cont->children))
> + if (!__memcg_has_children(memcg))
> memcg->use_hierarchy = val;
> else
> retval = -EBUSY;
> @@ -4917,8 +4943,7 @@ static int memcg_update_kmem_limit(struct cgroup *cont, u64 val)
> cgroup_lock();
> mutex_lock(&set_limit_mutex);
> if (!memcg->kmem_account_flags && val != RESOURCE_MAX) {
> - if (cgroup_task_count(cont) || (memcg->use_hierarchy &&
> - !list_empty(&cont->children))) {
> + if (cgroup_task_count(cont) || memcg_has_children(memcg)) {
> ret = -EBUSY;
> goto out;
> }
> @@ -5334,8 +5359,7 @@ static int mem_cgroup_swappiness_write(struct cgroup *cgrp, struct cftype *cft,
> cgroup_lock();
>
> /* If under hierarchy, only empty-root can set this value */
> - if ((parent->use_hierarchy) ||
> - (memcg->use_hierarchy && !list_empty(&cgrp->children))) {
> + if ((parent->use_hierarchy) || memcg_has_children(memcg)) {
> cgroup_unlock();
> return -EINVAL;
> }
> --
> 1.8.1
>
--
Michal Hocko
SUSE Labs
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