Re: [RFC PATCH] sched/core: Fix premature p->migration_pending completion

[Valentin Schneider <valentin.schneider@arm.com>]

On 04/02/21 15:30, Qais Yousef wrote:
> On 02/03/21 18:59, Valentin Schneider wrote:
>> On 03/02/21 17:23, Qais Yousef wrote:
>> > On 01/27/21 19:30, Valentin Schneider wrote:
>> >>   Initial conditions:
>> >>     victim.cpus_mask = {CPU0, CPU1}
>> >>
>> >>   CPU0                             CPU1                             CPU<don't care>
>> >>
>> >>   switch_to(victim)
>> >>                                                               set_cpus_allowed(victim, {CPU1})
>> >>                                                                 kick CPU0 migration_cpu_stop({.dest_cpu = CPU1})
>> >>   switch_to(stopper/0)
>> >>                                                               // e.g. CFS load balance
>> >>                                                               move_queued_task(CPU0, victim, CPU1);
>> >>                              switch_to(victim)
>> >>                                                               set_cpus_allowed(victim, {CPU0});
>> >>                                                                 task_rq_unlock();
>> >>   migration_cpu_stop(dest_cpu=CPU1)
>> >
>> > This migration stop is due to set_cpus_allowed(victim, {CPU1}), right?
>> >
>>
>> Right
>>
>> >>     task_rq(p) != rq && pending
>> >>       kick CPU1 migration_cpu_stop({.dest_cpu = CPU1})
>> >>
>> >>                              switch_to(stopper/1)
>> >>                              migration_cpu_stop(dest_cpu=CPU1)
>> >
>> > And this migration stop is due to set_cpus_allowed(victim, {CPU0}), right?
>> >
>>
>> Nein! This is a retriggering of the "current" stopper (triggered by
>> set_cpus_allowed(victim, {CPU1})), see the tail of that
>>
>>   else if (dest_cpu < 0 || pending)
>>
>> branch in migration_cpu_stop(), is what I'm trying to hint at with that
>>
>> task_rq(p) != rq && pending
>
> Okay I see. But AFAIU, the work will be queued in order. So we should first
> handle the set_cpus_allowed_ptr(victim, {CPU0}) before the retrigger, no?
>
> So I see migration_cpu_stop() running 3 times
>
>       1. because of set_cpus_allowed(victim, {CPU1}) on CPU0
>       2. because of set_cpus_allowed(victim, {CPU0}) on CPU1
>       3. because of retrigger of '1' on CPU0
>

On that 'CPU<don't care>' lane, I intentionally included task_rq_unlock()
but not 'kick CPU1 migration_cpu_stop({.dest_cpu = CPU0})'. IOW, there is
nothing in that trace that queues a stopper work for 2. - it *will* happen
at some point, but harm will already have been done.

The migrate_task_to() example is potentially worse, because it doesn't rely
on which stopper work gets enqueued first - only that an extra affinity
change happens before the first stopper work grabs the pi_lock and completes.