Messages in this thread | | | Subject | Re: [PATCH 2/4] sched/deadline: Improve admission control for asymmetric CPU capacities | From | Dietmar Eggemann <> | Date | Thu, 9 Apr 2020 19:29:45 +0200 |
| |
On 08.04.20 17:01, Valentin Schneider wrote: > > 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.
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); }
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.
| |