On Tue, Aug 27, 2024 at 4:11 PM Kinsey Ho <kinseyho@xxxxxxxxxx> wrote:
>
> To obtain the pointer to the next memcg position, mem_cgroup_iter()
> currently holds css->refcnt during memcg traversal only to put
> css->refcnt at the end of the routine. This isn't necessary as an
> rcu_read_lock is already held throughout the function. The use of
> the RCU read lock with css_next_descendant_pre() guarantees that
> sibling linkage is safe without holding a ref on the passed-in @css.
>
> Remove css->refcnt usage during traversal by leveraging RCU.
>
> Signed-off-by: Kinsey Ho <kinseyho@xxxxxxxxxx>
Reviewed-by: T.J. Mercier <tjmercier@xxxxxxxxxx>
I found a different place where a more trivial css get/put pair than
this could be removed, but I couldn't measure a perf difference. Like
Yosry, I appreciate the simplicity gains here though.
> ---
> mm/memcontrol.c | 18 +-----------------
> 1 file changed, 1 insertion(+), 17 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 35431035e782..67b1994377b7 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1013,20 +1013,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
> else if (reclaim->generation != iter->generation)
> goto out_unlock;
>
> - while (1) {
> - pos = READ_ONCE(iter->position);
> - if (!pos || css_tryget(&pos->css))
> - break;
> - /*
> - * css reference reached zero, so iter->position will
> - * be cleared by ->css_released. However, we should not
> - * rely on this happening soon, because ->css_released
> - * is called from a work queue, and by busy-waiting we
> - * might block it. So we clear iter->position right
> - * away.
> - */
> - (void)cmpxchg(&iter->position, pos, NULL);
> - }
> + pos = READ_ONCE(iter->position);
> } else if (prev) {
> pos = prev;
> }
> @@ -1067,9 +1054,6 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
> */
> (void)cmpxchg(&iter->position, pos, memcg);
>
> - if (pos)
> - css_put(&pos->css);
> -
> if (!memcg)
> iter->generation++;
> }
> --
> 2.46.0.295.g3b9ea8a38a-goog
>
>
