linux-kernel - [PATCH] mm: Remove the redundant updating of stats_flush

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Date:   Sat, 23 Jul 2022 00:49:49 +0800
From:   Jiebin Sun <jiebin.sun@...el.com>
To:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        cgroups@...r.kernel.org
Cc:     hannes@...xchg.org, mhocko@...nel.org, roman.gushchin@...ux.dev,
        shakeelb@...gle.com, songmuchun@...edance.com,
        akpm@...ux-foundation.org, tim.c.chen@...el.com,
        ying.huang@...el.com, amadeuszx.slawinski@...ux.intel.com,
        tianyou.li@...el.com, wangyang.guo@...el.com,
        jiebin sun <jiebin.sun@...el.com>
Subject: [PATCH] mm: Remove the redundant updating of stats_flush_threshold

From: jiebin sun <jiebin.sun@...el.com>

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)

Signed-off-by: Jiebin Sun <jiebin.sun@...el.com>
---
 mm/memcontrol.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index abec50f31fe6..9e8c6f24c694 100644
--- a/mm/memcontrol.c
+++ b/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);
 	}
 }
-- 
2.31.1