Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH] sched/fair: Remove the duplicate check from group_has_capacity() | Date | Tue, 11 Aug 2020 13:48:09 +0100 |
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On 11/08/20 12:44, Qi Zheng wrote: > On 2020/8/11 下午6:38, Valentin Schneider wrote: >> >> On 11/08/20 04:39, Qi Zheng wrote: >>> On 2020/8/11 上午2:33, Valentin Schneider wrote: >>>> >>>> On 10/08/20 02:00, Qi Zheng wrote: >>>>> 1. The group_has_capacity() function is only called in >>>>> group_classify(). >>>>> 2. The following inequality has already been checked in >>>>> group_is_overloaded() which was also called in >>>>> group_classify(). >>>>> >>>>> (sgs->group_capacity * imbalance_pct) < >>>>> (sgs->group_runnable * 100) >>>>> >>>> >>>> Consider group_is_overloaded() returns false because of the first >>>> condition: >>>> >>>> if (sgs->sum_nr_running <= sgs->group_weight) >>>> return false; >>>> >>>> then group_has_capacity() would be the first place where the group_runnable >>>> vs group_capacity comparison would be done. >>>> >>>> Now in that specific case we'll actually only check it if >>>> >>>> sgs->sum_nr_running == sgs->group_weight >>>> >>>> and the only case where the runnable vs capacity check can fail here is if >>>> there's significant capacity pressure going on. TBH this capacity pressure >>>> could be happening even when there are fewer tasks than CPUs, so I'm not >>>> sure how intentional that corner case is. >>> >>> Maybe some cpus in sg->cpumask are no longer active at the == case, >>> which causes the significant capacity pressure? >>> >> >> That can only happen in that short window between deactivating a CPU and >> not having rebuilt the sched_domains yet, which sounds quite elusive. >> > > In fact, at the beginning, I added unlikely() here to hint the compiler: > > - if ((sgs->group_capacity * imbalance_pct) < > - (sgs->group_runnable * 100)) > + if (unlikely((sgs->group_capacity * imbalance_pct) < > + (sgs->group_runnable * 100))) > > The corresponding patch is as follows: > > [PATCH]sched/core: add unlikely in group_has_capacity() > > Do you think it is necessary?
The "unlikely" approach has the benefit of keeping all corner cases in place. I was tempted to say it could still make sense to get rid of the extra check entirely, given that it has an impact only when:
- sum_nr_running == group_weight - group capacity has been noticeably reduced
If sum_nr_running < group_weight, we won't evaluate it. If sum_nr_running > group_weight, we either won't call into group_has_capacity() or we'll have checked it already in group_overloaded().
That said, it does make very much sense to check it in that == case. Vincent might have a different take on this, but right now I'd say the unlikely approach is the safest one of the two.
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