Messages in this thread | | | Date | Fri, 21 Oct 2022 19:14:47 +0800 | From | Chen Yu <> | Subject | Re: [PATCH v6 3/4] sched/fair: Introduce SIS_CORE |
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On 2022-10-21 at 17:35:06 +0800, Abel Wu wrote: > On 10/21/22 12:34 PM, Chen Yu wrote: > > On 2022-10-21 at 12:30:56 +0800, Abel Wu wrote: > > > Hi Chen, thanks for your reviewing! > > > > > > On 10/21/22 12:03 PM, Chen Yu wrote: > > > > On 2022-10-19 at 20:28:58 +0800, Abel Wu wrote: > > > > [cut] > > > > > A major concern is the accuracy of the idle cpumask. A cpu present > > > > > in the mask might not be idle any more, which is called the false > > > > > positive cpu. Such cpus will negate lots of benefit this feature > > > > > brings. The strategy against the false positives will be introduced > > > > > in next patch. > > > > > > > > > I was thinking that, if patch[3/4] needs [4/4] to fix the false positives, > > > > maybe SIS_CORE could be disabled by default in 3/4 but enabled > > > > in 4/4? So this might facilicate git bisect in case of any regression > > > > check? > > > > > > Agreed. Will fix in next version. > > > > > > > [cut] > > > > > + * To honor the rule of CORE granule update, set this cpu to the LLC idle > > > > > + * cpumask only if there is no cpu of this core showed up in the cpumask. > > > > > + */ > > > > > +static void update_idle_cpu(int cpu) > > > > > +{ > > > > > + struct sched_domain_shared *sds; > > > > > + > > > > > + if (!sched_feat(SIS_CORE)) > > > > > + return; > > > > > + > > > > > + sds = rcu_dereference(per_cpu(sd_llc_shared, cpu)); > > > > > + if (sds) { > > > > > + struct cpumask *icpus = to_cpumask(sds->icpus); > > > > > + > > > > > + /* > > > > > + * This is racy against clearing in select_idle_cpu(), > > > > > + * and can lead to idle cpus miss the chance to be set to > > > > > + * the idle cpumask, thus the idle cpus are temporarily > > > > > + * out of reach in SIS domain scan. But it should be rare > > > > > + * and we still have ILB to kick them working. > > > > > + */ > > > > > + if (!cpumask_intersects(cpu_smt_mask(cpu), icpus)) > > > > > + cpumask_set_cpu(cpu, icpus); > > > > Maybe I miss something, here we only set one CPU in the icpus, but > > > > when we reach update_idle_cpu(), all SMT siblings of 'cpu' are idle, > > > > is this intended for 'CORE granule update'? > > > > > > The __update_idle_core() is called by all the cpus that need to go idle > > > to update has_idle_core if necessary, and update_idle_cpu() is called > > > before that check. > > > > > I see. > > > > Since __update_idle_core() has checked all SMT siblings of 'cpu' if > > they are idle, can that information also be updated to icpus? > > I think this will simply fallback to the original per-cpu proposal and > lose the opportunity to spread tasks to different cores. OK, make sense.
thanks, Chenyu
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