Re: [PATCH v2] sched/fair: Fix per-CPU kthread and wakee stacking for asym CPU capacity

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 29.11.21 11:54, Valentin Schneider wrote:
> On 25/11/21 10:12, Vincent Donnefort wrote:
>> select_idle_sibling() has a special case for tasks woken up by a per-CPU
>> kthread. For this case, the chosen CPU is the previous one. This is an
>> issue for asymmetric CPU capacity systems where the wakee might not fit
>> that CPU anymore. Evaluate asym_fits_capacity() for prev_cpu before using
>> the exit path described above.
>>
>> Fixes: b4c9c9f15649 ("sched/fair: Prefer prev cpu in asymmetric wakeup path")
>> Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>
> 
> Per our discussion on v1, the asym check was intentionally omitted, the
> assumption being: we'd be putting p back on its prev CPU, its utilization
> cannot be bigger now than it was then so it should still pass the capacity
> fitness criterion (unless we dequeued it right before crossing the next
> PELT window boundary would have made it cross the tipping point...)

... and assume ~0 sleep time for p so sync_entity_load_avg() can't decay
p's util_avg.

> 
> Uclamp goes against this, p's uclamp.min can completely change between its
> dequeue and wakeup, which warrants adding the check.
> 
> I'd like to see (at least some of) the above in the changelog, but

+1 (since otherwise the WHY the wakee could potentially not fit on prev
anymore is hard to grasp).

> pedantism aside:
>> Reviewed-by: Valentin Schneider <Valentin.Schneider@arm.com>

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>

> 
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 6291876a9d32..b90dc6fd86ca 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -6410,7 +6410,8 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>>        */
>>       if (is_per_cpu_kthread(current) &&
>>           prev == smp_processor_id() &&
>> -	    this_rq()->nr_running <= 1) {
>> +	    this_rq()->nr_running <= 1 &&
>> +	    asym_fits_capacity(task_util, prev)) {
>>               return prev;
>>       }
>>
>> --
>> 2.25.1