Messages in this thread | | | Date | Thu, 22 Nov 2018 14:56:06 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v9 13/15] sched/fair: Introduce an energy estimation helper function |
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On Wed, Nov 21, 2018 at 04:05:27PM +0000, Quentin Perret wrote: > On Wednesday 21 Nov 2018 at 15:28:03 (+0100), Peter Zijlstra wrote: > > On Mon, Nov 19, 2018 at 02:18:55PM +0000, Quentin Perret wrote: > > > +static long > > > +compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd) > > > +{ > > > + long util, max_util, sum_util, energy = 0; > > > + int cpu; > > > + > > > + for (; pd; pd = pd->next) { > > > + max_util = sum_util = 0; > > > + /* > > > + * The capacity state of CPUs of the current rd can be driven by > > > + * CPUs of another rd if they belong to the same performance > > > + * domain. So, account for the utilization of these CPUs too > > > + * by masking pd with cpu_online_mask instead of the rd span. > > > + * > > > + * If an entire performance domain is outside of the current rd, > > > + * it will not appear in its pd list and will not be accounted > > > + * by compute_energy(). > > > + */ > > > + for_each_cpu_and(cpu, perf_domain_span(pd), cpu_online_mask) { > > > > Should that not be cpu_active_mask ? > > Hmm, I must admit I'm sometimes a bit confused by the exact difference > between these masks, so maybe yeah ... > > IIUC, cpu_active_mask is basically the set of CPUs on which the > scheduler is actually allowed to migrate tasks. Is that correct ?
Yep. Which is a strict subset of online. The difference only matters during hotplug. We take a CPU out of active before we take if offline and we add it to active only after the CPU is fully online and scheduling.
> I have always seen cpu_online_mask as a superset of cpu_active_mask > which can also include CPUs which are still running 'special' tasks > (kthreads and things like that I assume) although not allowed for > migration any more (or not yet) because we're in the process of > hotplugging that CPU.
Right.
> So, the thing is, I'm not trying to select a CPU candidate for my task > here, I'm trying to understand what's the energy impact of a migration. > That involves all CPUs that are running _something_ in a perf domain > no matter if they're allowed to run more tasks or not. I mean, raising > the OPP will make running online && !active CPUs more expensive as well. > That's why I thought cpu_online_mask was good match here.
Ah, fair enough. Thanks!
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