Messages in this thread Patch in this message | | | From | Quentin Perret <> | Subject | [PATCH v5 07/14] sched/topology: Introduce sched_energy_present static key | Date | Tue, 24 Jul 2018 13:25:14 +0100 |
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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).
The static key is checked in the rd_freq_domain() function which returns the frequency domains of a root domain when they are available. As EAS cannot be enabled with CONFIG_ENERGY_MODEL=n, rd_freq_domain() is stubbed to 'NULL' to let the compiler remove the unused EAS code by constant propagation.
cc: Ingo Molnar <mingo@redhat.com> cc: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Quentin Perret <quentin.perret@arm.com> --- kernel/sched/sched.h | 17 ++++++++++ kernel/sched/topology.c | 73 ++++++++++++++++++++++++++++++++++++++++- 2 files changed, 89 insertions(+), 1 deletion(-)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 25d64a0b6fe0..a317457804dd 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -2222,8 +2222,25 @@ static inline unsigned long cpu_util_irq(struct rq *rq) #ifdef CONFIG_SMP #ifdef CONFIG_ENERGY_MODEL +extern struct static_key_false sched_energy_present; +/** + * rd_freq_domain - Get the frequency domains of a root domain. + * + * Must be called from a RCU read-side critical section. + */ +static inline struct freq_domain *rd_freq_domain(struct root_domain *rd) +{ + if (!static_branch_unlikely(&sched_energy_present)) + return NULL; + + return rcu_dereference(rd->fd); +} #define freq_domain_span(fd) (to_cpumask(((fd)->obj->cpus))) #else +static inline struct freq_domain *rd_freq_domain(struct root_domain *rd) +{ + return NULL; +} #define freq_domain_span(fd) NULL #endif #endif diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c index 8f3f746b0d5e..483bb7bf7af6 100644 --- a/kernel/sched/topology.c +++ b/kernel/sched/topology.c @@ -275,12 +275,32 @@ static void destroy_freq_domain_rcu(struct rcu_head *rp) free_fd(fd); } +/* + * The complexity of the Energy Model is defined as: nr_fd * (nr_cpus + nr_cs) + * with: 'nr_fd' the number of frequency domains; 'nr_cpus' the number of CPUs; + * and 'nr_cs' the sum of the capacity states numbers of all frequency 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. + */ +#define EM_MAX_COMPLEXITY 2048 + + static void build_freq_domains(const struct cpumask *cpu_map) { + int i, nr_fd = 0, nr_cs = 0, nr_cpus = cpumask_weight(cpu_map); struct freq_domain *fd = 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_ea, cpu)) { + if (sched_debug()) + pr_info("rd %*pbl: !sd_ea\n", cpumask_pr_args(cpu_map)); + goto free; + } for_each_cpu(i, cpu_map) { /* Skip already covered CPUs. */ @@ -293,6 +313,18 @@ static void build_freq_domains(const struct cpumask *cpu_map) goto free; tmp->next = fd; fd = tmp; + + /* Count freq. doms and perf states for the complexity check. */ + nr_fd++; + nr_cs += em_fd_nr_cap_states(fd->obj); + } + + /* Bail out if the Energy Model complexity is too high. */ + if (nr_fd * (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; } freq_domain_debug(cpu_map, fd); @@ -312,6 +344,44 @@ static void build_freq_domains(const struct cpumask *cpu_map) if (tmp) call_rcu(&tmp->rcu, destroy_freq_domain_rcu); } + +/* + * 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); + +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 frequency domains. + */ + while (ndoms_new) { + if (cpu_rq(cpumask_first(doms_new[ndoms_new - 1]))->rd->fd) + goto enable; + ndoms_new--; + } + + if (static_branch_unlikely(&sched_energy_present)) { + 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_rd_fd(struct root_domain *rd) { } #endif @@ -2046,6 +2116,7 @@ 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: */ -- 2.18.0
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