Messages in this thread | | | Date | Wed, 29 Aug 2018 18:20:06 +0100 | From | Quentin Perret <> | Subject | Re: [PATCH v6 07/14] sched/topology: Introduce sched_energy_present static key |
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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
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