Re: [RFC] sched/fair: Use wake_q length as a hint for wake_wide

[Joel Fernandes]

On Wed, Sep 20, 2017 at 2:33 AM, Brendan Jackman
<brendan.jackman@xxxxxxx> wrote:
>
> On Wed, Sep 20 2017 at 05:06, Joel Fernandes wrote:
>>> On Tue, Sep 19, 2017 at 3:05 AM, Brendan Jackman
>>> <brendan.jackman@xxxxxxx> wrote:
>>>> On Mon, Sep 18 2017 at 22:15, Joel Fernandes wrote:
>> [..]
>>>>>> IIUC, if wake_affine() behaves correctly this trick wouldn't be
>>>>>> necessary on SMP systems, so it might be best guarded by the presence
>>>>>
>>>>> Actually wake_affine doesn't check for balance if previous/next cpu
>>>>> are within the same shared cache domain. The difference is some time
>>>>> ago it would return true for shared cache but now it returns false as
>>>>> of 4.14-rc1:
>>>>> http://elixir.free-electrons.com/linux/v4.14-rc1/source/kernel/sched/fair.c#L5466
>>>>>
>>>>> Since it would return false in the above wake up cases for task 1 and
>>>>> 2, it would then run select_idle_sibling on the previous CPU which is
>>>>> also within the big cluster, so I don't think it will make a
>>>>> difference in this case... Infact what it returns probably doesn't
>>>>> matter.
>>>>
>>>> So my paragraph here was making a leap in reasoning, let me try to fill
>>>> the gap: On SMP these tasks never need to move around. If by some chance
>>>> they did get coscheduled, the first load balance would spread them out and
>>>> then every time they wake up from then on, prev_cpu is the sensible
>>>> choice. So it will look something like:
>>>>
>>>>             v CPU v           ->time->
>>>>
>>>>                             -------------
>>>>          {  0  (SAME)       11111111111
>>>>   cache  {                  -------------
>>>>          {  1  (SAME)       222222222222|
>>>>                             -------------
>>>>          {  2  (SAME)       33333333333
>>>>   cache  {                  -------------
>>>>          {  3  (SAME)       44444444444
>>>>                             -------------
>>>>
>>>> So here, task 2 wakes up the other guys and when it's doing tasks 3 and
>>>> 4, prev_cpu and smp_processor_id() don't share a cache, so IIUC its'
>>>> basically wake_affine's job to decide between prev_cpu and
>>>> smp_processor_id(). So "if wake_affine is behaving correctly" the
>>>> problem that this patch aims to solve (i.e. the fact that we overload
>>>> the waker's LLC domain because of bias towards prev_cpu) does not arise
>>>> on SMP.
>>>>
>>>
>>> Yes SMP, but your patch is for solving a problem for non-SMP. So your
>>> original statement about wake_affine solving any problem for SMP is
>>> not relevant I feel :-P. I guess you can just kill this para from the
>>> commit message to prevent confusion.
>>
>> Ok I take that back, you were talking about guarding this feature by
>> the SD_ASYM_CPUCAPACITY flag.
>>
>> I don't think that protection would be helpful because you can have
>> the same issue if the tasks do different amount of work on SMP. So in
>> that case some threads might still complete before the others and you
>> run into the same thing.
>
> Well assuming we're still talking about one task per CPU, if you have
> tasks doing different amount of work there's still no reason to move the
> longer-running threads around. The only reason that happens in my
> example is because of the asym capacity.

Yes but you can very well have RT pressure and things that temporarily
change the capacity equality. Also this is a simple benchmark and for
any reason you have more than 1 task running on those other CPUs and
then the idle CPUs run some of the tasks and you run into a similar
situation that might need your patch..

Also one more note, the SD_ASYM_CPUCAPACITY protection is still not
needed because SD_BALANCE_WAKE isn't turned on for
!SD_ASYM_CPUCAPACITY from what I learnt from discussions with Mike, I
believe its this piece of code in sd_init that actually enables it:

        if (sd->flags & SD_ASYM_CPUCAPACITY) {
                struct sched_domain *t = sd;

                for_each_lower_domain(t)
                        t->flags |= SD_BALANCE_WAKE;
        }


thanks,

- Joel