memcg-optimize-memcg_rstat_updated.patch removed from -mm tree

The quilt patch titled
Subject: memcg: optimize memcg_rstat_updated
has been removed from the -mm tree. Its filename was
memcg-optimize-memcg_rstat_updated.patch

This patch was dropped because it was merged into the mm-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: Shakeel Butt <[email protected]>
Subject: memcg: optimize memcg_rstat_updated
Date: Wed, 9 Apr 2025 19:57:52 -0700

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.

---

 mm/memcontrol.c |   16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

@@ -592,18 +592,20 @@ static inline void memcg_rstat_updated(s
 	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);
 	}
 }

_

Patches currently in -mm which might be from [email protected] are

memcg-introduce-non-blocking-limit-setting-option.patch memcg-introduce-non-blocking-limit-setting-option-v3.patch memcg-simplify-consume_stock.patch memcg-separate-local_trylock-for-memcg-and-obj.patch memcg-completely-decouple-memcg-and-obj-stocks.patch memcg-no-irq-disable-for-memcg-stock-lock.patch