memcg: do not flush stats in irq context

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.

Link: https://lkml.kernel.org/r/20230330191801.1967435-4-yosryahmed@google.com


Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Suggested-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Tejun Heo <tj@kernel.org>
Cc: Vasily Averin <vasily.averin@linux.dev>
Cc: Zefan Li <lizefan.x@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>