Messages in this thread |  | | Subject | Re: [PATCH 8/9] sched/fair: select idle cpu from idle cpumask for task wakeup | From | Aubrey Li <> | Date | Thu, 5 Aug 2021 08:23:15 +0800 |
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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
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