memcg: optimize memcg_rstat_updated

Currently the kernel maintains the stats updates per-memcg which is needed
to implement stats flushing threshold.  On the update side, the update is
added to the per-cpu per-memcg update of the given memcg and all of its
ancestors.  However when the given memcg has passed the flushing
threshold, all of its ancestors should have passed the threshold as well.
There is no need to traverse up the memcg tree to maintain the stats
updates.

Perf profile collected from our fleet shows that memcg_rstat_updated is
one of the most expensive memcg function i.e.  a lot of cumulative CPU is
being spent on it.  So, even small micro optimizations matter a lot.  This
patch is microbenchmarked with multiple instances of netperf on a single
machine with locally running netserver and we see couple of percentage of
improvement.

Link: https://lkml.kernel.org/r/20250410025752.92159-1-shakeel.butt@linux.dev
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index aebb1f2c8657b572155436916526b50df49ad146..e64ac7942a21d51dc186aea282ceabc872f4794a 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -592,18 +592,20 @@ static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
        cgroup_rstat_updated(memcg->css.cgroup, cpu);
        statc = this_cpu_ptr(memcg->vmstats_percpu);
        for (; statc; statc = statc->parent) {
+               /*
+                * If @memcg is already flushable then all its ancestors are
+                * flushable as well and also there is no need to increase
+                * stats_updates.
+                */
+               if (memcg_vmstats_needs_flush(statc->vmstats))
+                       break;
+
                stats_updates = READ_ONCE(statc->stats_updates) + abs(val);
                WRITE_ONCE(statc->stats_updates, stats_updates);
                if (stats_updates < MEMCG_CHARGE_BATCH)
                        continue;
 
-               /*
-                * If @memcg is already flush-able, increasing stats_updates is
-                * redundant. Avoid the overhead of the atomic update.
-                */
-               if (!memcg_vmstats_needs_flush(statc->vmstats))
-                       atomic64_add(stats_updates,
-                                    &statc->vmstats->stats_updates);
+               atomic64_add(stats_updates, &statc->vmstats->stats_updates);
                WRITE_ONCE(statc->stats_updates, 0);
        }
 }