On Wednesday 29 Aug 2018 at 17:50:58 (+0100), Patrick Bellasi wrote:
> > +/*
> > + * The complexity of the Energy Model is defined as: nr_pd * (nr_cpus +
> > nr_cs)
> > + * with: 'nr_pd' the number of performance domains; 'nr_cpus' the number of
> > + * CPUs; and 'nr_cs' the sum of the capacity states numbers of all
> > performance
> > + * domains.
> > + *
> > + * It is generally not a good idea to use such a model in the wake-up path
> > on
> > + * very complex platforms because of the associated scheduling overheads.
> > The
> > + * arbitrary constraint below prevents that. It makes EAS usable up to 16
> > CPUs
> > + * with per-CPU DVFS and less than 8 capacity states each, for example.
>
> According to the formula above, that should give a "complexity value" of:
>
> 16 * (16 + 9) = 384
>
> while, 2K complexity seems more like a 40xCPUs system with 8 OPPs.
>
> Maybe we should update either the example or the constant below ?
Hmm I guess the example isn't really clear. 'nr_cs' is the _sum_ of the
number of OPPs of all perf. domains. So, in the example above, if you
have 16 CPUs with per-CPU DVFS, and each DVFS island has 8 OPPs, then
nr_cs = 16 * 8 = 128.
So if you apply the formula you get C = 16 * (16 + 128) = 2304, which is
more than EM_MAX_COMPLEXITY, so EAS cannot start.
If the DVFS island had 7 OPPs instead of 8 (for example) you would get
nr_cs = 112, C = 2048, and so EAS could start.
I can try to re-work that comment to explain things a bit better ...
>
> > + */
> > +#define EM_MAX_COMPLEXITY 2048
> > +
> > static void build_perf_domains(const struct cpumask *cpu_map)
> > {
> > + int i, nr_pd = 0, nr_cs = 0, nr_cpus = cpumask_weight(cpu_map);
> > struct perf_domain *pd = NULL, *tmp;
> > int cpu = cpumask_first(cpu_map);
> > struct root_domain *rd = cpu_rq(cpu)->rd;
> > - int i;
> > +
> > + /* EAS is enabled for asymmetric CPU capacity topologies. */
> > + if (!per_cpu(sd_asym_cpucapacity, cpu)) {
> > + if (sched_debug()) {
> > + pr_info("rd %*pbl: CPUs do not have asymmetric
> > capacities\n",
> > + cpumask_pr_args(cpu_map));
> > + }
> > + goto free;
> > + }
> >
> > for_each_cpu(i, cpu_map) {
> > /* Skip already covered CPUs. */
> > @@ -288,6 +318,21 @@ static void build_perf_domains(const struct cpumask
> > *cpu_map)
> > goto free;
> > tmp->next = pd;
> > pd = tmp;
> > +
> > + /*
> > + * Count performance domains and capacity states for the
> > + * complexity check.
> > + */
> > + nr_pd++;
>
> A special case where EAS is not going to be used is for systems where
> nr_pd matches the number of online CPUs, isn't it ?
Well, it depends. Say you have only 4 CPUs with 3 OPPs each. Even with
per-CPU DVFS the complexity is low enough to start EAS. I don't really
see a good reason for not doing so no ?
>
> If that's the case, then, by caching this nr_pd you can probably check
> this condition in the sched_energy_start() and bail out even faster by
> avoiding to scan all the doms_new's pd ?
>
>
> > + nr_cs += em_pd_nr_cap_states(pd->obj);
> > + }
> > +
> > + /* Bail out if the Energy Model complexity is too high. */
> > + if (nr_pd * (nr_cs + nr_cpus) > EM_MAX_COMPLEXITY) {
> > + if (sched_debug())
> > + pr_info("rd %*pbl: EM complexity is too high\n ",
> > + cpumask_pr_args(cpu_map));
> > + goto free;
> > }
> >
> > perf_domain_debug(cpu_map, pd);
> > @@ -307,6 +352,35 @@ static void build_perf_domains(const struct cpumask
> > *cpu_map)
> > if (tmp)
> > call_rcu(&tmp->rcu, destroy_perf_domain_rcu);
> > }
> > +
> > +static void sched_energy_start(int ndoms_new, cpumask_var_t doms_new[])
> > +{
> > + /*
> > + * The conditions for EAS to start are checked during the creation of
> > + * root domains. If one of them meets all conditions, it will have a
> > + * non-null list of performance domains.
> > + */
> > + while (ndoms_new) {
> > + if (cpu_rq(cpumask_first(doms_new[ndoms_new - 1]))->rd->pd)
> > + goto enable;
> > + ndoms_new--;
> > + }
> > +
> > + if (static_branch_unlikely(&sched_energy_present)) {
> ^^^^^^^^
> Is this defined unlikely to reduce overheads on systems which never
> satisfy all the conditions above while still rebuild SDs from time to
> time ?
Something like that. I just thought that the case where EAS needs to be
disabled after being enabled isn't very common. I mean, the most typical
use-case is, EAS is enabled at boot and stays enabled forever, or EAS
never gets enabled.
Enabling/disabling EAS because of hotplug (for example) can definitely
happen, but that shouldn't be the case very often in practice, I think.
So we can optimize things out a bit I suppose.
Thanks !
Quentin
