On 03/02/21 17:23, Qais Yousef wrote:
> On 01/27/21 19:30, Valentin Schneider wrote:
>> Fiddling some more with a TLA+ model of set_cpus_allowed_ptr() & friends
>> unearthed one more outstanding issue. This doesn't even involve
>> migrate_disable(), but rather affinity changes and execution of the stopper
>> racing with each other.
>>
>> My own interpretation of the (lengthy) TLA+ splat (note the potential for
>> errors at each level) is:
>>
>> 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
> If I didn't miss something, then dest_cpu should be CPU0 too, not CPU1 and the
> task should be moved back to CPU0 as expected?
>
> Thanks
>
> --
> Qais Yousef
