Re: [PATCH v5 29/32] mm: memcontrol: prepare for reparenting non-hierarchical stats

[Yosry Ahmed <yosry@kernel.org>]

> @@ -473,6 +493,29 @@ unsigned long lruvec_page_state_local(struct lruvec *lruvec,
>         return x;
>  }
>
> +#ifdef CONFIG_MEMCG_V1
> +void reparent_memcg_lruvec_state_local(struct mem_cgroup *memcg,
> +                                      struct mem_cgroup *parent, int idx)
> +{
> +       int i = memcg_stats_index(idx);
> +       int nid;
> +
> +       if (WARN_ONCE(BAD_STAT_IDX(i), "%s: missing stat item %d\n", __func__, idx))
> +               return;
> +
> +       for_each_node(nid) {
> +               struct lruvec *child_lruvec = mem_cgroup_lruvec(memcg, NODE_DATA(nid));
> +               struct lruvec *parent_lruvec = mem_cgroup_lruvec(parent, NODE_DATA(nid));
> +               struct mem_cgroup_per_node *parent_pn;
> +               unsigned long value = lruvec_page_state_local(child_lruvec, idx);
> +
> +               parent_pn = container_of(parent_lruvec, struct mem_cgroup_per_node, lruvec);
> +
> +               atomic_long_add(value, &(parent_pn->lruvec_stats->state_local[i]));
> +       }
> +}

Did you measure the impact of making state_local atomic on the flush
path? It's a slow path but we've seen pain from it being too slow
before, because it extends the critical section of the rstat flush
lock.

Can we keep this non-atomic and use mod_memcg_lruvec_state() here? It
will update the stat on the local counter and it will be added to
state_local in the flush path when needed. We can even force another
flush in reparent_state_local () after reparenting is completed, if we
want to avoid leaving a potentially large stat update pending, as it
can be missed by mem_cgroup_flush_stats_ratelimited().

Same for reparent_memcg_state_local(), we can probably use mod_memcg_state()?