Re: [PATCH 2/4] sched/deadline: Improve admission control for asymmetric CPU capacities

[Valentin Schneider <valentin.schneider@arm.com>]

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).