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SubjectRe: [PATCH v3 1/3] sched/fair: Add asymmetric CPU capacity wakeup scan
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On 29/01/2020 11:04, Dietmar Eggemann wrote:
>> + /*
>> + * It would be silly to keep looping when we've found a CPU
>> + * of highest available capacity. Just check that it's not been
>> + * too pressured lately.
>> + */
>> + if (rq->cpu_capacity_orig == READ_ONCE(rq->rd->max_cpu_capacity) &&
>
> There is a similar check in check_misfit_status(). Common helper function?

Mright, and check_misfit_status() is missing the READ_ONCE(). That said...

>
>> + !check_cpu_capacity(rq, sd))
>> + return cpu;
>
> I wonder how this special treatment of a big CPU behaves in (LITTLE,
> medium, big) system like Pixel4 (Snapdragon 855):
>
> flame:/ $ cat /sys/devices/system/cpu/cpu*/cpu_capacity
>
> 261
> 261
> 261
> 261
> 871
> 871
> 871
> 1024
>
> Or on legacy systems where the sd->imbalance_pct is 25% instead of 17%?
>

... This is a very valid point. When I wrote this bit I had the good old
big.LITTLE split in mind where there are big differences between the capacity
values. As you point out, that's not so true with DynamIQ systems sporting
> 2 capacity values. The issue here is that we could bail early picking a
(slightly) pressured big (1024 capacity_orig) when there was a non-pressured
idle medium (871 capacity orig).

It's borderline in this example - the threshold for a big to be seen as
pressured by check_cpu_capacity(), assuming a flat topology with just an MC
domain, is ~ 875. If we have e.g. mediums at 900 and bigs at 1024, this
logic is broken.

So this is pretty much a case of my trying to be too clever for my own good,
I'll remove that "fastpath" in v4. Thanks for pointing it out!

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