Re: [PATCH 8/9] sched/fair: select idle cpu from idle cpumask for task wakeup

[Aubrey Li]

On 8/4/21 6:26 PM, Mel Gorman wrote:
> On Mon, Aug 02, 2021 at 10:41:13AM +0000, Song Bao Hua (Barry Song) wrote:
>> Hi Mel, Aubrey,
>> A similar thing Yicong and me has discussed is having a mask or a count for
>> idle cores. And then we can only scan idle cores in this mask in
>> select_idle_cpu().
>>
> 
> Either approach would require a lot of updates.
> 
>> A obvious problem is that has_idle_cores is a bool, it can seriously lag
>> from the real status. I mean, after system enters the status without idle
>> cores, has_idle_cores could be still true.
>>
> 
> True.
> 
>> Right now, we are setting has_idle_cores to true while cpu enters idle
>> and its smt sibling is also idle. But we are setting has_idle_cores to
>> false only after we scan all cores in a llc.
>>
>> So we have thought for a while to provide an idle core mask. But never
>> really made a workable patch.
>>
>> Mel's patch7/9 limits the number of cores which will be scanned in
>> select_idle_cpu(), it might somehow alleviate the problem we redundantly
>> scan all cores while we actually have no idle core even has_idle_cores
>> is true.
>>
> 
> I prototyped a patch that tracked the location of a known idle core and
> use it as a starting hint for a search. It's cleared if the core is
> selected for use. Unfortunately, it was not a universal win so was
> dropped for the moment but either way, improving the accurate of
> has_idle_cores without being too expensive would be niuce.

The idle core information is already in idle cpu mask, do we have a quick
and cheap way to extract out?


> 
>> However, if we can get idle core mask, it could be also good to
>> select_idle_core()? Maybe after that, we don't have to enforce
>> proportional scan limits while scanning for an idle core?
>>
> 
> To remove the limits entirely, it would have to be very accurate and
> it's hard to see how that can be done without having a heavily updated
> shared cache line.
> 

Bad idea, is it helpful to make a sparse cpu mask for this? :p

Thanks,
-Aubrey