Re: [RFC][PATCH 15/16] sched: Trivial forced-newidle balancer

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

On 21/02/2019 16:47, Peter Zijlstra wrote:
[...]
>>> IIUC, we're trying to find/steal tasks matching the core_cookie from other
>>> rqs because dst has been cookie-forced-idle.
>>>
>>> If the p we find isn't running, what's the meaning of core_occupation?
>>> I would have expected it to be 0, but we don't seem to be clearing it when
>>> resetting the state in pick_next_task().
>>
>> Indeed. We preserve the occupation from the last time around; it's not
>> perfect but its better than nothing.
>>
>> Consider there's two groups; and we just happen to run the other group.
>> Then our occopation, being what it was last, is still accurate. When
>> next we run, we'll again get that many siblings together.
>>
>>> If it is running, we prevent the stealing if the core it's on is running
>>> more matching tasks than the core of the pulling rq. It feels to me as if
>>> that's a balancing tweak to try to cram as many matching tasks as possible
>>> in a single core, so to me this reads as "don't steal my tasks if I'm
>>> running more than you are, but I will steal tasks from you if I'm given
>>> the chance". Is that correct?
>>
>> Correct, otherwise an SMT4 with 5 tasks could end up ping-ponging the
>> one task forever.
>>

Wouldn't we want to move some tasks in those cases? If we're going newidle
we're guaranteed to have a thread for that extra task.

So

  if (p->core_occupation == cpumask_weight(cpu_smt_mask(that))

we could want to steal, overriding the occupation comparison
(we already have a (p == src->core_pick) abort before). Kind of feels like
CFS stealing that steals when nr_running > 1.

>> Note that a further condition a little up the callchain from here only
>> does this stealing if the thread was forced-idle -- ie. it had something
>> to run anyway. So under the condition where there simple aren't enough
>> tasks to keep all siblings busy, we'll not compact just cause.
> 
> Better example; it will avoid stealing a task from a full SMT2 core to
> fill another.
> 

Aye, that's the scenario I was thinking of.

Thanks for clearing things up.