[v5,6/6] mm/memcg: Disable migration instead of preemption in drain_all_stock().

Commit Message

Sebastian Andrzej Siewior Feb. 25, 2022, 8:13 p.m. UTC

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 <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
---
v4…v5: Added Michal's notes regarding CPU hotplug.

 mm/memcontrol.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

Patch

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 3d7ccb104374c..9c29b1a0e6bec 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2293,7 +2293,8 @@  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;
@@ -2316,7 +2317,7 @@  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);
 }