mm/memcontrol.c: remove the redundant updating of stats_flush_threshold

Remove the redundant updating of stats_flush_threshold.  If the global var
stats_flush_threshold has exceeded the trigger value for
__mem_cgroup_flush_stats, further increment is unnecessary.

Apply the patch and test the pts/hackbench-1.0.0 Count:4 (160 threads).

Score gain: 1.95x
Reduce CPU cycles in __mod_memcg_lruvec_state (44.88% -> 0.12%)

CPU: ICX 8380 x 2 sockets
Core number: 40 x 2 physical cores
Benchmark: pts/hackbench-1.0.0 Count:4 (160 threads)

Link: https://lkml.kernel.org/r/20220722164949.47760-1-jiebin.sun@intel.com
Signed-off-by: Jiebin Sun <jiebin.sun@intel.com>
Acked-by: Shakeel Butt <shakeelb@google.com>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
Acked-by: Muchun Song <songmuchun@bytedance.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Amadeusz Sawiski <amadeuszx.slawinski@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/memcontrol.c

@@ -626,7 +626,14 @@ static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
 
 	x = __this_cpu_add_return(stats_updates, abs(val));
 	if (x > MEMCG_CHARGE_BATCH) {
-		atomic_add(x / MEMCG_CHARGE_BATCH, &stats_flush_threshold);
+		/*
+		 * If stats_flush_threshold exceeds the threshold
+		 * (>num_online_cpus()), cgroup stats update will be triggered
+		 * in __mem_cgroup_flush_stats(). Increasing this var further
+		 * is redundant and simply adds overhead in atomic update.
+		 */
+		if (atomic_read(&stats_flush_threshold) <= num_online_cpus())
+			atomic_add(x / MEMCG_CHARGE_BATCH, &stats_flush_threshold);
 		__this_cpu_write(stats_updates, 0);
 	}
 }