From 675ed4d9ee37607741439653838f98b081cb9d44 Mon Sep 17 00:00:00 2001
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Mon, 8 Aug 2022 14:55:58 +0200
Subject: [PATCH 165/183] mm/memcg: Disable migration instead of preemption in
 drain_all_stock().

Before the for-each-CPU loop, preemption is disabled so that so that
drain_local_stock() can be invoked directly instead of scheduling a
worker. Ensuring that drain_local_stock() completed on the local CPU is
not correctness problem. It _could_ be that the charging path will be
forced to reclaim memory because cached charges are still waiting for
their draining.

Disabling preemption before invoking drain_local_stock() is problematic
on PREEMPT_RT due to the sleeping locks involved. To ensure that no CPU
migrations happens across for_each_online_cpu() it is enouhg to use
migrate_disable() which disables migration and keeps context preemptible
to a sleeping lock can be acquired.
A race with CPU hotplug is not a problem because pcp data is not going away.
In the worst case we just schedule draining of an empty stock.

Use migrate_disable() instead of get_cpu() around the
for_each_online_cpu() loop.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Michal Hocko <mhocko@suse.com>
[do: backported to v5.15]
Signed-off-by: David Oberhollenzer <goliath@infraroot.at>
Signed-off-by: Clark Williams <williams@redhat.com>
Signed-off-by: Clark Williams <clrkwllms@kernel.org>
---
 mm/memcontrol.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ff73f29a1ce6..61208b8b7122 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2316,7 +2316,8 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
 	 * as well as workers from this path always operate on the local
 	 * per-cpu data. CPU up doesn't touch memcg_stock at all.
 	 */
-	curcpu = get_cpu();
+	migrate_disable();
+	curcpu = smp_processor_id();
 	for_each_online_cpu(cpu) {
 		struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
 		struct mem_cgroup *memcg;
@@ -2339,7 +2340,7 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
 				schedule_work_on(cpu, &stock->work);
 		}
 	}
-	put_cpu();
+	migrate_enable();
 	mutex_unlock(&percpu_charge_mutex);
 }
 
-- 
2.47.1

