Hi Quentin,
a couple of minor notes/questions follow...
Best,
Patrick
On 20-Aug 10:44, Quentin Perret wrote:
> In order to ensure a minimal performance impact on non-energy-aware
> systems, introduce a static_key guarding the access to Energy-Aware
> Scheduling (EAS) code.
>
> The static key is set iff all the following conditions are met for at
> least one root domain:
> 1. all online CPUs of the root domain are covered by the Energy
> Model (EM);
> 2. the complexity of the root domain's EM is low enough to keep
> scheduling overheads low;
> 3. the root domain has an asymmetric CPU capacity topology (detected
> by looking for the SD_ASYM_CPUCAPACITY flag in the sched_domain
> hierarchy).
>
> cc: Ingo Molnar <mi...@redhat.com>
> cc: Peter Zijlstra <pet...@infradead.org>
> Signed-off-by: Quentin Perret <quentin.per...@arm.com>
> ---
> kernel/sched/sched.h | 1 +
> kernel/sched/topology.c | 77 ++++++++++++++++++++++++++++++++++++++++-
> 2 files changed, 77 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 4b884e467545..cb3d6afdb114 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1421,6 +1421,7 @@ static const_debug __maybe_unused unsigned int
> sysctl_sched_features =
>
> extern struct static_key_false sched_numa_balancing;
> extern struct static_key_false sched_schedstats;
> +extern struct static_key_false sched_energy_present;
>
> static inline u64 global_rt_period(void)
> {
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 4c6a36a8d7b8..1cb86a0ef00f 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -200,6 +200,14 @@ sd_parent_degenerate(struct sched_domain *sd, struct
> sched_domain *parent)
>
> return 1;
> }
> +/*
> + * This static_key is set if at least one root domain meets all the following
> + * conditions:
> + * 1. all CPUs of the root domain are covered by the EM;
> + * 2. the EM complexity is low enough to keep scheduling overheads low;
> + * 3. the SD_ASYM_CPUCAPACITY flag is set in the sched_domain hierarchy.
> + */
> +DEFINE_STATIC_KEY_FALSE(sched_energy_present);
>
> #ifdef CONFIG_ENERGY_MODEL
> static void free_pd(struct perf_domain *pd)
> @@ -270,12 +278,34 @@ static void destroy_perf_domain_rcu(struct rcu_head *rp)
> free_pd(pd);
> }
>
> +/*
> + * 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 ?
> + */
> +#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 ?
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 ?
> + if (sched_debug())
> + pr_info("%s: stopping EAS\n", __func__);
> + static_branch_disable_cpuslocked(&sched_energy_present);
> + }
> +
> + return;
> +
> +enable:
> + if (!static_branch_unlikely(&sched_energy_present)) {
> + if (sched_debug())
> + pr_info("%s: starting EAS\n", __func__);
> + static_branch_enable_cpuslocked(&sched_energy_present);
> + }
> +}
> #else
> static void free_pd(struct perf_domain *pd) { }
> #endif
> @@ -2123,6 +2197,7 @@ 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: */
> --
> 2.17.1
>
--
#include <best/regards.h>
Patrick Bellasi
