linux-kernel - [PATCH] memcg: decouple memcg_hotplug_cpu_dead from stock

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Message-ID: <20250410210623.1016767-1-shakeel.butt@linux.dev>
Date: Thu, 10 Apr 2025 14:06:23 -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>,
	Yosry Ahmed <yosry.ahmed@...ux.dev>,
	Waiman Long <llong@...hat.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	linux-mm@...ck.org,
	cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Meta kernel team <kernel-team@...a.com>
Subject: [PATCH] memcg: decouple memcg_hotplug_cpu_dead from stock_lock

The function memcg_hotplug_cpu_dead works on the stock of a remote dead
CPU and drain_obj_stock works on the given stock instead of local stock,
so there is no need to take local stock_lock anymore.

Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>
---
 mm/memcontrol.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f23a4d0ad239..2178a051bd09 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1789,7 +1789,7 @@ static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock) = {
 };
 static DEFINE_MUTEX(percpu_charge_mutex);
 
-static void drain_obj_stock(struct memcg_stock_pcp *stock);
+static void __drain_obj_stock(struct memcg_stock_pcp *stock);
 static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
 				     struct mem_cgroup *root_memcg);
 
@@ -1873,7 +1873,7 @@ static void drain_local_stock(struct work_struct *dummy)
 	local_lock_irqsave(&memcg_stock.stock_lock, flags);
 
 	stock = this_cpu_ptr(&memcg_stock);
-	drain_obj_stock(stock);
+	__drain_obj_stock(stock);
 	drain_stock(stock);
 	clear_bit(FLUSHING_CACHED_CHARGE, &stock->flags);
 
@@ -1964,10 +1964,10 @@ static int memcg_hotplug_cpu_dead(unsigned int cpu)
 
 	stock = &per_cpu(memcg_stock, cpu);
 
-	/* drain_obj_stock requires stock_lock */
-	local_lock_irqsave(&memcg_stock.stock_lock, flags);
-	drain_obj_stock(stock);
-	local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
+	local_irq_save(flag);
+	/* stock of a remote dead cpu, no need for stock_lock. */
+	__drain_obj_stock(stock);
+	local_irq_restore(flag);
 
 	drain_stock(stock);
 
@@ -2837,7 +2837,11 @@ static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
 	return ret;
 }
 
-static void drain_obj_stock(struct memcg_stock_pcp *stock)
+/*
+ * Works on the given stock. The callers are responsible for the proper locking
+ * for the local or remote stocks.
+ */
+static void __drain_obj_stock(struct memcg_stock_pcp *stock)
 {
 	struct obj_cgroup *old = READ_ONCE(stock->cached_objcg);
 
@@ -2925,7 +2929,7 @@ static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes,
 
 	stock = this_cpu_ptr(&memcg_stock);
 	if (READ_ONCE(stock->cached_objcg) != objcg) { /* reset if necessary */
-		drain_obj_stock(stock);
+		__drain_obj_stock(stock);
 		obj_cgroup_get(objcg);
 		stock->nr_bytes = atomic_read(&objcg->nr_charged_bytes)
 				? atomic_xchg(&objcg->nr_charged_bytes, 0) : 0;
-- 
2.47.1