Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH 2/4] sched/deadline: Improve admission control for asymmetric CPU capacities | Date | Wed, 08 Apr 2020 16:01:43 +0100 |
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On 08/04/20 14:30, luca abeni wrote: >> >> I don't think this is strictly equivalent to what we have now for the >> SMP case. 'cpus' used to come from dl_bw_cpus(), which is an ugly way >> of writing >> >> cpumask_weight(rd->span AND cpu_active_mask); >> >> The rd->cpu_capacity_orig field you added gets set once per domain >> rebuild, so it also happens in sched_cpu_(de)activate() but is >> separate from touching cpu_active_mask. AFAICT this mean we can >> observe a CPU as !active but still see its capacity_orig accounted in >> a root_domain. > > Sorry, I suspect this is my fault, because the bug comes from my > original patch. > When I wrote the original code, I believed that when a CPU is > deactivated it is also removed from its root domain. > > I now see that I was wrong. >
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.
> > Luca
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