Messages in this thread | | | Subject | Re: [PATCH] sched/fair: Remove the duplicate check from group_has_capacity() | From | Qi Zheng <> | Date | Tue, 11 Aug 2020 19:44:34 +0800 |
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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cpumask are no longer active at the == case, 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?
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