Re: [patch 3/3] mm: memcontrol: drop bogus RCU locking from mem_cgroup_same_or_subtree()

Vladimir Davydov
2014-10-24T15:22:51Z

On Fri, Oct 24, 2014 at 09:49:49AM -0400, Johannes Weiner wrote:
> None of the mem_cgroup_same_or_subtree() callers actually require it
> to take the RCU lock, either because they hold it themselves or they
> have css references. Remove it.

It seems we don't need these rcu_read_lock/unlock there since commit
b47f77b5a224 ("memcg: convert to use cgroup_is_descendant()"), which
removed RCU-ish css_is_ancestor() from mem_cgroup_same_or_subtree().

Reviewed-by: Vladimir Davydov <vdavydov@parallels.com>

>
> To make the API change clear, rename the leftover helper to
> mem_cgroup_is_descendant() to match cgroup_is_descendant().
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
> include/linux/memcontrol.h | 13 +++++-----
> mm/memcontrol.c | 59 +++++++++++++---------------------------------
> mm/oom_kill.c | 4 ++--
> 3 files changed, 24 insertions(+), 52 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index e32ab948f589..d4575a1d6e99 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -68,10 +68,9 @@ void mem_cgroup_migrate(struct page *oldpage, struct page *newpage,
> struct lruvec *mem_cgroup_zone_lruvec(struct zone *, struct mem_cgroup *);
> struct lruvec *mem_cgroup_page_lruvec(struct page *, struct zone *);
>
> -bool __mem_cgroup_same_or_subtree(const struct mem_cgroup *root_memcg,
> - struct mem_cgroup *memcg);
> -bool task_in_mem_cgroup(struct task_struct *task,
> - const struct mem_cgroup *memcg);
> +bool mem_cgroup_is_descendant(struct mem_cgroup *memcg,
> + struct mem_cgroup *root);
> +bool task_in_mem_cgroup(struct task_struct *task, struct mem_cgroup *memcg);
>
> extern struct mem_cgroup *try_get_mem_cgroup_from_page(struct page *page);
> extern struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p);
> @@ -79,8 +78,8 @@ extern struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p);
> extern struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *memcg);
> extern struct mem_cgroup *mem_cgroup_from_css(struct cgroup_subsys_state *css);
>
> -static inline
> -bool mm_match_cgroup(const struct mm_struct *mm, const struct mem_cgroup *memcg)
> +static inline bool mm_match_cgroup(struct mm_struct *mm,
> + struct mem_cgroup *memcg)
> {
> struct mem_cgroup *task_memcg;
> bool match = false;
> @@ -88,7 +87,7 @@ bool mm_match_cgroup(const struct mm_struct *mm, const struct mem_cgroup *memcg)
> rcu_read_lock();
> task_memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
> if (task_memcg)
> - match = __mem_cgroup_same_or_subtree(memcg, task_memcg);
> + match = mem_cgroup_is_descendant(task_memcg, memcg);
> rcu_read_unlock();
> return match;
> }
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 15b1c5110a8f..f75b92a44ac6 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1307,41 +1307,24 @@ void mem_cgroup_update_lru_size(struct lruvec *lruvec, enum lru_list lru,
> VM_BUG_ON((long)(*lru_size) < 0);
> }
>
> -/*
> - * Checks whether given mem is same or in the root_mem_cgroup's
> - * hierarchy subtree
> - */
> -bool __mem_cgroup_same_or_subtree(const struct mem_cgroup *root_memcg,
> - struct mem_cgroup *memcg)
> +bool mem_cgroup_is_descendant(struct mem_cgroup *memcg, struct mem_cgroup *root)
> {
> - if (root_memcg == memcg)
> + if (root == memcg)
> return true;
> - if (!root_memcg->use_hierarchy)
> + if (!root->use_hierarchy)
> return false;
> - return cgroup_is_descendant(memcg->css.cgroup, root_memcg->css.cgroup);
> -}
> -
> -static bool mem_cgroup_same_or_subtree(const struct mem_cgroup *root_memcg,
> - struct mem_cgroup *memcg)
> -{
> - bool ret;
> -
> - rcu_read_lock();
> - ret = __mem_cgroup_same_or_subtree(root_memcg, memcg);
> - rcu_read_unlock();
> - return ret;
> + return cgroup_is_descendant(memcg->css.cgroup, root->css.cgroup);
> }
>
> -bool task_in_mem_cgroup(struct task_struct *task,
> - const struct mem_cgroup *memcg)
> +bool task_in_mem_cgroup(struct task_struct *task, struct mem_cgroup *memcg)
> {
> - struct mem_cgroup *curr;
> + struct mem_cgroup *task_memcg;
> struct task_struct *p;
> bool ret;
>
> p = find_lock_task_mm(task);
> if (p) {
> - curr = get_mem_cgroup_from_mm(p->mm);
> + task_memcg = get_mem_cgroup_from_mm(p->mm);
> task_unlock(p);
> } else {
> /*
> @@ -1350,18 +1333,12 @@ bool task_in_mem_cgroup(struct task_struct *task,
> * killed to prevent needlessly killing additional tasks.
> */
> rcu_read_lock();
> - curr = mem_cgroup_from_task(task);
> - css_get(&curr->css);
> + task_memcg = mem_cgroup_from_task(task);
> + css_get(&task_memcg->css);
> rcu_read_unlock();
> }
> - /*
> - * We should check use_hierarchy of "memcg" not "curr". Because checking
> - * use_hierarchy of "curr" here make this function true if hierarchy is
> - * enabled in "curr" and "curr" is a child of "memcg" in *cgroup*
> - * hierarchy(even if use_hierarchy is disabled in "memcg").
> - */
> - ret = mem_cgroup_same_or_subtree(memcg, curr);
> - css_put(&curr->css);
> + ret = mem_cgroup_is_descendant(task_memcg, memcg);
> + css_put(&task_memcg->css);
> return ret;
> }
>
> @@ -1446,8 +1423,8 @@ static bool mem_cgroup_under_move(struct mem_cgroup *memcg)
> if (!from)
> goto unlock;
>
> - ret = mem_cgroup_same_or_subtree(memcg, from)
> - || mem_cgroup_same_or_subtree(memcg, to);
> + ret = mem_cgroup_is_descendant(from, memcg) ||
> + mem_cgroup_is_descendant(to, memcg);
> unlock:
> spin_unlock(&mc.lock);
> return ret;
> @@ -1813,12 +1790,8 @@ static int memcg_oom_wake_function(wait_queue_t *wait,
> oom_wait_info = container_of(wait, struct oom_wait_info, wait);
> oom_wait_memcg = oom_wait_info->memcg;
>
> - /*
> - * Both of oom_wait_info->memcg and wake_memcg are stable under us.
> - * Then we can use css_is_ancestor without taking care of RCU.
> - */
> - if (!mem_cgroup_same_or_subtree(oom_wait_memcg, wake_memcg)
> - && !mem_cgroup_same_or_subtree(wake_memcg, oom_wait_memcg))
> + if (!mem_cgroup_is_descendant(wake_memcg, oom_wait_memcg) &&
> + !mem_cgroup_is_descendant(oom_wait_memcg, wake_memcg))
> return 0;
> return autoremove_wake_function(wait, mode, sync, arg);
> }
> @@ -2138,7 +2111,7 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
> memcg = stock->cached;
> if (!memcg || !stock->nr_pages)
> continue;
> - if (!mem_cgroup_same_or_subtree(root_memcg, memcg))
> + if (!mem_cgroup_is_descendant(memcg, root_memcg))
> continue;
> if (!test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags)) {
> if (cpu == curcpu)
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 3348280eef89..864bba992735 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -119,7 +119,7 @@ found:
>
> /* return true if the task is not adequate as candidate victim task. */
> static bool oom_unkillable_task(struct task_struct *p,
> - const struct mem_cgroup *memcg, const nodemask_t *nodemask)
> + struct mem_cgroup *memcg, const nodemask_t *nodemask)
> {
> if (is_global_init(p))
> return true;
> @@ -353,7 +353,7 @@ static struct task_struct *select_bad_process(unsigned int *ppoints,
> * State information includes task's pid, uid, tgid, vm size, rss, nr_ptes,
> * swapents, oom_score_adj value, and name.
> */
> -static void dump_tasks(const struct mem_cgroup *memcg, const nodemask_t *nodemask)
> +static void dump_tasks(struct mem_cgroup *memcg, const nodemask_t *nodemask)
> {
> struct task_struct *p;
> struct task_struct *task;