linux-kernel - [RFC PATCH 7/7] memcg: no stock lock for cpu hot-unplug

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Message-ID: <20250513031316.2147548-8-shakeel.butt@linux.dev>
Date: Mon, 12 May 2025 20:13:16 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...nel.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Muchun Song <muchun.song@...ux.dev>,
	Vlastimil Babka <vbabka@...e.cz>,
	Alexei Starovoitov <ast@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Harry Yoo <harry.yoo@...cle.com>,
	Yosry Ahmed <yosry.ahmed@...ux.dev>,
	bpf@...r.kernel.org,
	linux-mm@...ck.org,
	cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Meta kernel team <kernel-team@...a.com>
Subject: [RFC PATCH 7/7] memcg: no stock lock for cpu hot-unplug

Previously on the cpu hot-unplug, the kernel would call
drain_obj_stock() with objcg local lock. However local lock was not
neede as the stock which was accessed belongs to a dead cpu but we kept
it there to disable irqs as drain_obj_stock() may call
mod_objcg_mlstate() which required irqs disabled. However there is no
need to disable irqs now for mod_objcg_mlstate(), so we can remove the
lcoal lock altogether from cpu hot-unplug path.

Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>
---
 mm/memcontrol.c | 10 +---------
 1 file changed, 1 insertion(+), 9 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index af7df675d733..539cd76e1492 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2060,16 +2060,8 @@ void drain_all_stock(struct mem_cgroup *root_memcg)
 
 static int memcg_hotplug_cpu_dead(unsigned int cpu)
 {
-	struct obj_stock_pcp *obj_st;
-
-	obj_st = &per_cpu(obj_stock, cpu);
-
-	/* drain_obj_stock requires objstock.lock */
-	local_lock(&obj_stock.lock);
-	drain_obj_stock(obj_st);
-	local_unlock(&obj_stock.lock);
-
 	/* no need for the local lock */
+	drain_obj_stock(&per_cpu(obj_stock, cpu));
 	drain_stock_fully(&per_cpu(memcg_stock, cpu));
 
 	return 0;
-- 
2.47.1