Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH 2/4] sched/deadline: Improve admission control for asymmetric CPU capacities | Date | Tue, 14 Apr 2020 15:28:08 +0100 |
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On 09/04/20 18:29, Dietmar Eggemann wrote: >> Well it is indeed the case, but sadly it's not an atomic step - AFAICT with >> cpusets we do hold some cpuset lock when calling __dl_overflow() and when >> rebuilding the domains, but not when fiddling with the active mask. >> >> I just realized it's even more obvious for dl_cpu_busy(): IIUC it is meant >> to prevent the removal of a CPU if it would lead to a DL overflow - it >> works now because the active mask is modified before it gets called, but >> here it breaks because it's called before the sched_domain rebuild. >> >> Perhaps re-computing the root domain capacity sum at every dl_bw_cpus() >> call would be simpler. It's a bit more work, but then we already have a >> for_each_cpu_*() loop, and we only rely on the masks being correct. > > Maybe we can do a hybrid. We have rd->span and rd->sum_cpu_capacity and > with the help of an extra per-cpu cpumask we could just > > DEFINE_PER_CPU(cpumask_var_t, dl_bw_mask); > > dl_bw_cpus(int i) { > > struct cpumask *cpus = this_cpu_cpumask_var_ptr(dl_bw_mask); > ... > cpumask_and(cpus, rd->span, cpu_active_mask); > > return cpumask_weight(cpus);
+1 on making this use cpumask_weight() :)
> } > > and > > dl_bw_capacity(int i) { > > struct cpumask *cpus = this_cpu_cpumask_var_ptr(dl_bw_mask); > ... > cpumask_and(cpus, rd->span, cpu_active_mask); > if (cpumask_equal(cpus, rd->span)) > return rd->sum_cpu_capacity; > > for_each_cpu(i, cpus) > cap += capacity_orig_of(i); > > return cap; > } > > So only in cases in which rd->span and cpu_active_mask differ we would > have to sum up again.
I think this might just work. In the "stable" case (i.e. not racing with hotplug), we can use the value cached in the root_domain. Otherwise we'll detect the mismatch between the cpumask and the root_domain (i.e. CPU active but not yet included in root_domain, or CPU !active but still included in root_domain).
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