On Tue 28-03-23 22:16:38, Yosry Ahmed wrote:
> Currently, the only context in which we can invoke an rstat flush from
> irq context is through mem_cgroup_usage() on the root memcg when called
> from memcg_check_events(). An rstat flush is an expensive operation that
> should not be done in irq context, so do not flush stats and use the
> stale stats in this case.
>
> Arguably, usage threshold events are not reliable on the root memcg
> anyway since its usage is ill-defined.
>
> Suggested-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Suggested-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> Acked-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> mm/memcontrol.c | 16 +++++++++++++++-
> 1 file changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index c3b6aae78901..ff39f78f962e 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3669,7 +3669,21 @@ static unsigned long mem_cgroup_usage(struct mem_cgroup *memcg, bool swap)
> unsigned long val;
>
> if (mem_cgroup_is_root(memcg)) {
> - mem_cgroup_flush_stats();
> + /*
> + * We can reach here from irq context through:
> + * uncharge_batch()
> + * |--memcg_check_events()
> + * |--mem_cgroup_threshold()
> + * |--__mem_cgroup_threshold()
> + * |--mem_cgroup_usage
> + *
> + * rstat flushing is an expensive operation that should not be
> + * done from irq context; use stale stats in this case.
> + * Arguably, usage threshold events are not reliable on the root
> + * memcg anyway since its usage is ill-defined.
> + */
> + if (in_task())
> + mem_cgroup_flush_stats();
> val = memcg_page_state(memcg, NR_FILE_PAGES) +
> memcg_page_state(memcg, NR_ANON_MAPPED);
> if (swap)
> --
> 2.40.0.348.gf938b09366-goog
--
Michal Hocko
SUSE Labs
