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

[Mel Gorman <mgorman@techsingularity.net>]

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.

> 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.

-- 
Mel Gorman
SUSE Labs