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SubjectRe: [PATCH v9 09/15] sched: Introduce sched_energy_present static key
On Wednesday 21 Nov 2018 at 15:14:45 (+0000), Quentin Perret wrote:
> Hi Peter,
>
> On Wednesday 21 Nov 2018 at 14:08:22 (+0100), Peter Zijlstra wrote:
> > On Mon, Nov 19, 2018 at 02:18:51PM +0000, Quentin Perret wrote:
> >
> > > +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--;
> > > + }
> >
> > That seems quite ugly; can't you simply return a bool from
> > build_perf_domains() ?
> >
> > Something like the below, but less fugly ;-)
> >
> > --- a/kernel/sched/topology.c
> > +++ b/kernel/sched/topology.c
> > @@ -299,7 +299,7 @@ static void destroy_perf_domain_rcu(stru
> > #define EM_MAX_COMPLEXITY 2048
> >
> > extern struct cpufreq_governor schedutil_gov;
> > -static void build_perf_domains(const struct cpumask *cpu_map)
> > +static bool 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;
> > @@ -365,7 +365,7 @@ static void build_perf_domains(const str
> > if (tmp)
> > call_rcu(&tmp->rcu, destroy_perf_domain_rcu);
> >
> > - return;
> > + return !!pd;
> >
> > free:
> > free_pd(pd);
> > @@ -373,6 +373,8 @@ static void build_perf_domains(const str
> > rcu_assign_pointer(rd->pd, NULL);
> > if (tmp)
> > call_rcu(&tmp->rcu, destroy_perf_domain_rcu);
> > +
> > + return false;
> > }
> > #else
> > static void free_pd(struct perf_domain *pd) { }
> > @@ -2173,6 +2175,9 @@ void partition_sched_domains(int ndoms_n
> > }
> >
> > #if defined(CONFIG_ENERGY_MODEL) && defined(CONFIG_CPU_FREQ_GOV_SCHEDUTIL)
> > + {
> > + bool has_eas = false;
>
> That one wants to be declared '__maybe_unused' at the top of
> partition_sched_domains() perhaps ? Just to avoid the { } section.
> Hopefully the compiler will be smart enough to not use stack space for
> it if not needed.
>
> > +
> > /* Build perf. domains: */
> > for (i = 0; i < ndoms_new; i++) {
> > for (j = 0; j < n && !sched_energy_update; j++) {
> > @@ -2181,10 +2186,13 @@ void partition_sched_domains(int ndoms_n
> > goto match3;
>
> And we want to do 'has_eas = true' just before 'goto match3' here.
> Otherwise we'll end up disabling EAS if we hit 'goto match3' for all
> root domains.
>
> > }
> > /* No match - add perf. domains for a new rd */
> > - build_perf_domains(doms_new[i]);
> > + has_eas |= build_perf_domains(doms_new[i]);
> > match3:
> > ;
> > }
> > +
> > + sched_energy_set(has_eas);
> > + }
> > #endif
> >
> > /* Remember the new sched domains: */
>
> Other than that I guess that should work OK :-)

So I actually just came across your patch that does
static_branch_{inc,dec} on sched_smt_present. Maybe I could do something
similar here ?

How about something like the below (totally untested):

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 8f9dfea60344..10ac32a0dc2e 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -298,6 +298,7 @@ static void destroy_perf_domain_rcu(struct rcu_head *rp)

pd = container_of(rp, struct perf_domain, rcu);
free_pd(pd);
+ static_branch_dec_cpuslocked(&sched_energy_present);
}

static void sched_energy_start(int ndoms_new, cpumask_var_t doms_new[])
@@ -421,6 +422,8 @@ static void build_perf_domains(const struct cpumask *cpu_map)
/* Attach the new list of performance domains to the root domain. */
tmp = rd->pd;
rcu_assign_pointer(rd->pd, pd);
+ static_branch_inc_cpuslocked(&sched_energy_present);
+
if (tmp)
call_rcu(&tmp->rcu, destroy_perf_domain_rcu);

@@ -2246,7 +2249,6 @@ void partition_sched_domains(int ndoms_new, cpumask_var_t doms_new[],
match3:
;
}
- sched_energy_start(ndoms_new, doms_new);
#endif

/* Remember the new sched domains: */
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