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

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

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