From: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> Subject: 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. Link: https://lkml.kernel.org/r/20220226204144.1008339-7-bigeasy@xxxxxxxxxxxxx Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> Acked-by: Michal Hocko <mhocko@xxxxxxxx> Cc: Johannes Weiner <hannes@xxxxxxxxxxx> Cc: kernel test robot <oliver.sang@xxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxxxx> Cc: Michal Koutný <mkoutny@xxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx> Cc: Waiman Long <longman@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/memcontrol.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) --- a/mm/memcontrol.c~mm-memcg-disable-migration-instead-of-preemption-in-drain_all_stock +++ a/mm/memcontrol.c @@ -2300,7 +2300,8 @@ static void drain_all_stock(struct mem_c * 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_c schedule_work_on(cpu, &stock->work); } } - put_cpu(); + migrate_enable(); mutex_unlock(&percpu_charge_mutex); } _
