Messages in this thread | | | Subject | Re: [PATCH 06/10] sched/fair: Clear the target CPU from the cpumask of CPUs searched | From | "Li, Aubrey" <> | Date | Fri, 4 Dec 2020 21:47:28 +0800 |
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On 2020/12/4 21:40, Li, Aubrey wrote: > On 2020/12/4 21:17, Vincent Guittot wrote: >> On Fri, 4 Dec 2020 at 14:13, Vincent Guittot <vincent.guittot@linaro.org> wrote: >>> >>> On Fri, 4 Dec 2020 at 12:30, Mel Gorman <mgorman@techsingularity.net> wrote: >>>> >>>> On Fri, Dec 04, 2020 at 11:56:36AM +0100, Vincent Guittot wrote: >>>>>> The intent was that the sibling might still be an idle candidate. In >>>>>> the current draft of the series, I do not even clear this so that the >>>>>> SMT sibling is considered as an idle candidate. The reasoning is that if >>>>>> there are no idle cores then an SMT sibling of the target is as good an >>>>>> idle CPU to select as any. >>>>> >>>>> Isn't the purpose of select_idle_smt ? >>>>> >>>> >>>> Only in part. >>>> >>>>> select_idle_core() looks for an idle core and opportunistically saves >>>>> an idle CPU candidate to skip select_idle_cpu. In this case this is >>>>> useless loops for select_idle_core() because we are sure that the core >>>>> is not idle >>>>> >>>> >>>> If select_idle_core() finds an idle candidate other than the sibling, >>>> it'll use it if there is no idle core -- it picks a busy sibling based >>>> on a linear walk of the cpumask. Similarly, select_idle_cpu() is not >>> >>> My point is that it's a waste of time to loop the sibling cpus of >>> target in select_idle_core because it will not help to find an idle >>> core. The sibling cpus will then be check either by select_idle_cpu >>> of select_idle_smt >> >> also, while looping the cpumask, the sibling cpus of not idle cpu are >> removed and will not be check >> > > IIUC, select_idle_core and select_idle_cpu share the same cpumask(select_idle_mask)? > If the target's sibling is removed from select_idle_mask from select_idle_core(), > select_idle_cpu() will lose the chance to pick it up?
aha, no, select_idle_mask will be re-assigned in select_idle_cpu() by:
cpumask_and(cpus, sds_idle_cpus(sd->shared), p->cpus_ptr);
So, yes, I guess we can remove the cpu_smt_mask(target) from select_idle_core() safely.
> > Thanks, > -Aubrey >
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