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/[email protected]
Signed-off-by: Yosry Ahmed <[email protected]>
Suggested-by: Johannes Weiner <[email protected]>
Suggested-by: Shakeel Butt <[email protected]>
Acked-by: Shakeel Butt <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Josef Bacik <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Michal Koutný <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Vasily Averin <[email protected]>
Cc: Zefan Li <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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