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

Sebastian Andrzej Siewior
2022-02-26T20:41:44Z

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-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
Acked-by: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>
---
mm/memcontrol.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6439b0089d392..89664d8094bc0 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2300,7 +2300,8 @@ 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;
@@ -2323,7 +2324,7 @@ 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);
}

--
2.35.1