Re: [PATCH v6 07/14] sched/topology: Introduce sched_energy_present static key

[Quentin Perret <quentin.perret@arm.com>]

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