One of our workloads (Postgres 14 + sysbench OLTP) regressed on newer
upstream kernel and on further investigation, it seems like the cause is
the always synchronous rstat flush in the count_shadow_nodes() added by
the commit f82e6bf9 ("mm: memcg: use rstat for non-hierarchical
stats").  On further inspection it seems like we don't really need
accurate stats in this function as it was already approximating the amount
of appropriate shadow entries to keep for maintaining the refault
information.  Since there is already 2 sec periodic rstat flush, we don't
need exact stats here.  Let's ratelimit the rstat flush in this code path.

Link: https://lkml.kernel.org/r/20231228073055.4046430-1-shakeelb@google.com


Fixes: f82e6bf9 ("mm: memcg: use rstat for non-hierarchical stats")
Signed-off-by: Shakeel Butt <shakeelb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>