On Wed 05-06-13 18:53:46, Johannes Weiner wrote:
> mem_cgroup_iter() is too hard to follow. Factor out the lockless
> reclaim iterator loading and updating so it's easier to follow the big
> picture.
>
> Also document the iterator invalidation mechanism a bit more
> extensively.
>
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
I like this
Acked-by: Michal Hocko <mhocko@xxxxxxx>
> ---
> mm/memcontrol.c | 86 ++++++++++++++++++++++++++++++++++++++-------------------
> 1 file changed, 57 insertions(+), 29 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index e2cbb44..23a9236 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1148,6 +1148,58 @@ skip_node:
> return NULL;
> }
>
> +static void mem_cgroup_iter_invalidate(struct mem_cgroup *root)
> +{
> + /*
> + * When a group in the hierarchy below root is destroyed, the
> + * hierarchy iterator can no longer be trusted since it might
> + * have pointed to the destroyed group. Invalidate it.
> + */
> + atomic_inc(&root->dead_count);
> +}
> +
> +static struct mem_cgroup *
> +mem_cgroup_iter_load(struct mem_cgroup_reclaim_iter *iter,
> + struct mem_cgroup *root,
> + int *sequence)
> +{
> + struct mem_cgroup *position = NULL;
> + /*
> + * A cgroup destruction happens in two stages: offlining and
> + * release. They are separated by a RCU grace period.
> + *
> + * If the iterator is valid, we may still race with an
> + * offlining. The RCU lock ensures the object won't be
> + * released, tryget will fail if we lost the race.
> + */
> + *sequence = atomic_read(&root->dead_count);
> + if (iter->last_dead_count == *sequence) {
> + smp_rmb();
> + position = iter->last_visited;
> + if (position && !css_tryget(&position->css))
> + position = NULL;
> + }
> + return position;
> +}
> +
> +static void mem_cgroup_iter_update(struct mem_cgroup_reclaim_iter *iter,
> + struct mem_cgroup *last_visited,
> + struct mem_cgroup *new_position,
> + int sequence)
> +{
> + if (last_visited)
> + css_put(&last_visited->css);
> + /*
> + * We store the sequence count from the time @last_visited was
> + * loaded successfully instead of rereading it here so that we
> + * don't lose destruction events in between. We could have
> + * raced with the destruction of @new_position after all.
> + */
> + iter->last_visited = new_position;
> + smp_wmb();
> + iter->last_dead_count = sequence;
> +}
> +
> /**
> * mem_cgroup_iter - iterate over memory cgroup hierarchy
> * @root: hierarchy root
> @@ -1171,7 +1223,6 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
> {
> struct mem_cgroup *memcg = NULL;
> struct mem_cgroup *last_visited = NULL;
> - unsigned long uninitialized_var(dead_count);
>
> if (mem_cgroup_disabled())
> return NULL;
> @@ -1191,6 +1242,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
> rcu_read_lock();
> while (!memcg) {
> struct mem_cgroup_reclaim_iter *uninitialized_var(iter);
> + int uninitialized_var(seq);
>
> if (reclaim) {
> int nid = zone_to_nid(reclaim->zone);
> @@ -1204,37 +1256,13 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
> goto out_unlock;
> }
>
> - /*
> - * If the dead_count mismatches, a destruction
> - * has happened or is happening concurrently.
> - * If the dead_count matches, a destruction
> - * might still happen concurrently, but since
> - * we checked under RCU, that destruction
> - * won't free the object until we release the
> - * RCU reader lock. Thus, the dead_count
> - * check verifies the pointer is still valid,
> - * css_tryget() verifies the cgroup pointed to
> - * is alive.
> - */
> - dead_count = atomic_read(&root->dead_count);
> - if (dead_count == iter->last_dead_count) {
> - smp_rmb();
> - last_visited = iter->last_visited;
> - if (last_visited &&
> - !css_tryget(&last_visited->css))
> - last_visited = NULL;
> - }
> + last_visited = mem_cgroup_iter_load(iter, root, &seq);
> }
>
> memcg = __mem_cgroup_iter_next(root, last_visited);
>
> if (reclaim) {
> - if (last_visited)
> - css_put(&last_visited->css);
> -
> - iter->last_visited = memcg;
> - smp_wmb();
> - iter->last_dead_count = dead_count;
> + mem_cgroup_iter_update(iter, last_visited, memcg, seq);
>
> if (!memcg)
> iter->generation++;
> @@ -6319,14 +6347,14 @@ static void mem_cgroup_invalidate_reclaim_iterators(struct mem_cgroup *memcg)
> struct mem_cgroup *parent = memcg;
>
> while ((parent = parent_mem_cgroup(parent)))
> - atomic_inc(&parent->dead_count);
> + mem_cgroup_iter_invalidate(parent);
>
> /*
> * if the root memcg is not hierarchical we have to check it
> * explicitely.
> */
> if (!root_mem_cgroup->use_hierarchy)
> - atomic_inc(&root_mem_cgroup->dead_count);
> + mem_cgroup_iter_invalidate(root_mem_cgroup);
> }
>
> static void mem_cgroup_css_offline(struct cgroup *cont)
> --
> 1.8.3
>
--
Michal Hocko
SUSE Labs
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