Re: [PATCH v3 6/7] sched/fair: Filter out locally-unsolvable misfit imbalances

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

On 19/03/21 16:19, Vincent Guittot wrote:
> On Mon, 15 Mar 2021 at 20:18, Valentin Schneider
> <valentin.schneider@arm.com> wrote:
>> As stated the current behaviour is to classify groups as group_misfit_task
>> regardless of the dst_cpu's capacity. When we see a group_misfit_task
>> candidate group misfit task with higher per-CPU capacity than the local
>> group, we don't pick it as busiest.
>>
>> I initially thought not marking those as group_misfit_task was the right
>> thing to do, as they could then be classified as group_fully_busy or
>> group_has_spare. Consider:
>>
>>   DIE [          ]
>>   MC  [    ][    ]
>>        0  1  2  3
>>        L  L  B  B
>>
>>   arch_scale_capacity(L) < arch_scale_capacity(B)
>>
>>   CPUs 0-1 are idle / lightly loaded
>>   CPU2 has a misfit task and a few very small tasks
>>   CPU3 has a few very small tasks
>>
>> When CPU0 is running load_balance() at DIE level, right now we'll classify
>> the [2-3] group as group_misfit_task and not pick it as busiest because the
>> local group has a lower CPU capacity.
>>
>> If we didn't do that, we could leave the misfit task alone and pull some
>> small task(s) from CPU2 or CPU3, which would be a good thing to
>
> Are you sure? the last check in update_sd_pick_busiest() should
> already filter this. So it should be enough to let it be classify
> correctly
>
> A group should be classified as group_misfit_task when there is a task
> to migrate in priority compared to some other groups. In your case,
> you tag it as group_misfit_task but in order to do the opposite, i.e.
> make sure to not select it. As mentioned above, this will be filter in
> the last check in update_sd_pick_busiest()
>

This hinges on sgc->min_capacity, which might be influenced by a CPU in the
candidate group being severely pressured by IRQ / thermal / RT / DL
pressure. That said, you have a point in that this check and the one in
find_busiest_queue() catches most scenarios I can think of.

Let me ponder about this some more, and if throw it at the test
infrastructure monster if I go down that route.