mm: memcg: change flush_next_time to flush_last_time

mm: memcg: change flush_next_time to flush_last_time

Patch series "mm: memcg: subtree stats flushing and thresholds", v4.

This series attempts to address shortages in today's approach for memcg
stats flushing, namely occasionally stale or expensive stat reads.  The
series does so by changing the threshold that we use to decide whether to
trigger a flush to be per memcg instead of global (patch 3), and then
changing flushing to be per memcg (i.e.  subtree flushes) instead of
global (patch 5).

This patch (of 5):

flush_next_time is an inaccurate name.  It's not the next time that
periodic flushing will happen, it's rather the next time that ratelimited
flushing can happen if the periodic flusher is late.

Simplify its semantics by just storing the timestamp of the last flush
instead, flush_last_time.  Move the 2*FLUSH_TIME addition to
mem_cgroup_flush_stats_ratelimited(), and add a comment explaining it.
This way, all the ratelimiting semantics live in one place.

No functional change intended.

Link: https://lkml.kernel.org/r/20231129032154.3710765-1-yosryahmed@google.com
Link: https://lkml.kernel.org/r/20231129032154.3710765-2-yosryahmed@google.com
Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Tested-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
Acked-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Chris Li <chrisl@kernel.org> (Google)
Tested-by: Bagas Sanjaya <bagasdotme@gmail.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: Ivan Babrou <ivan@cloudflare.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Koutny <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Tejun Heo <tj@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Wei Xu <weixugc@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>