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    Subject[PATCH v10 14/15] sched/fair: Select an energy-efficient CPU on task wake-up
    Date
    If an Energy Model (EM) is available and if the system isn't
    overutilized, re-route waking tasks into an energy-aware placement
    algorithm. The selection of an energy-efficient CPU for a task
    is achieved by estimating the impact on system-level active energy
    resulting from the placement of the task on the CPU with the highest
    spare capacity in each performance domain. This strategy spreads tasks
    in a performance domain and avoids overly aggressive task packing. The
    best CPU energy-wise is then selected if it saves a large enough amount
    of energy with respect to prev_cpu.

    Although it has already shown significant benefits on some existing
    targets, this approach cannot scale to platforms with numerous CPUs.
    This is an attempt to do something useful as writing a fast heuristic
    that performs reasonably well on a broad spectrum of architectures isn't
    an easy task. As such, the scope of usability of the energy-aware
    wake-up path is restricted to systems with the SD_ASYM_CPUCAPACITY flag
    set, and where the EM isn't too complex.

    Cc: Ingo Molnar <mingo@redhat.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Signed-off-by: Quentin Perret <quentin.perret@arm.com>
    ---
    kernel/sched/fair.c | 143 +++++++++++++++++++++++++++++++++++++++++++-
    1 file changed, 141 insertions(+), 2 deletions(-)

    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index a20018ad9236..d73e7db5976a 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -6453,6 +6453,137 @@ compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd)
    return energy;
    }

    +/*
    + * find_energy_efficient_cpu(): Find most energy-efficient target CPU for the
    + * waking task. find_energy_efficient_cpu() looks for the CPU with maximum
    + * spare capacity in each performance domain and uses it as a potential
    + * candidate to execute the task. Then, it uses the Energy Model to figure
    + * out which of the CPU candidates is the most energy-efficient.
    + *
    + * The rationale for this heuristic is as follows. In a performance domain,
    + * all the most energy efficient CPU candidates (according to the Energy
    + * Model) are those for which we'll request a low frequency. When there are
    + * several CPUs for which the frequency request will be the same, we don't
    + * have enough data to break the tie between them, because the Energy Model
    + * only includes active power costs. With this model, if we assume that
    + * frequency requests follow utilization (e.g. using schedutil), the CPU with
    + * the maximum spare capacity in a performance domain is guaranteed to be among
    + * the best candidates of the performance domain.
    + *
    + * In practice, it could be preferable from an energy standpoint to pack
    + * small tasks on a CPU in order to let other CPUs go in deeper idle states,
    + * but that could also hurt our chances to go cluster idle, and we have no
    + * ways to tell with the current Energy Model if this is actually a good
    + * idea or not. So, find_energy_efficient_cpu() basically favors
    + * cluster-packing, and spreading inside a cluster. That should at least be
    + * a good thing for latency, and this is consistent with the idea that most
    + * of the energy savings of EAS come from the asymmetry of the system, and
    + * not so much from breaking the tie between identical CPUs. That's also the
    + * reason why EAS is enabled in the topology code only for systems where
    + * SD_ASYM_CPUCAPACITY is set.
    + *
    + * NOTE: Forkees are not accepted in the energy-aware wake-up path because
    + * they don't have any useful utilization data yet and it's not possible to
    + * forecast their impact on energy consumption. Consequently, they will be
    + * placed by find_idlest_cpu() on the least loaded CPU, which might turn out
    + * to be energy-inefficient in some use-cases. The alternative would be to
    + * bias new tasks towards specific types of CPUs first, or to try to infer
    + * their util_avg from the parent task, but those heuristics could hurt
    + * other use-cases too. So, until someone finds a better way to solve this,
    + * let's keep things simple by re-using the existing slow path.
    + */
    +
    +static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
    +{
    + unsigned long prev_energy = ULONG_MAX, best_energy = ULONG_MAX;
    + struct root_domain *rd = cpu_rq(smp_processor_id())->rd;
    + int cpu, best_energy_cpu = prev_cpu;
    + struct perf_domain *head, *pd;
    + unsigned long cpu_cap, util;
    + struct sched_domain *sd;
    +
    + rcu_read_lock();
    + pd = rcu_dereference(rd->pd);
    + if (!pd || READ_ONCE(rd->overutilized))
    + goto fail;
    + head = pd;
    +
    + /*
    + * Energy-aware wake-up happens on the lowest sched_domain starting
    + * from sd_asym_cpucapacity spanning over this_cpu and prev_cpu.
    + */
    + sd = rcu_dereference(*this_cpu_ptr(&sd_asym_cpucapacity));
    + while (sd && !cpumask_test_cpu(prev_cpu, sched_domain_span(sd)))
    + sd = sd->parent;
    + if (!sd)
    + goto fail;
    +
    + sync_entity_load_avg(&p->se);
    + if (!task_util_est(p))
    + goto unlock;
    +
    + for (; pd; pd = pd->next) {
    + unsigned long cur_energy, spare_cap, max_spare_cap = 0;
    + int max_spare_cap_cpu = -1;
    +
    + for_each_cpu_and(cpu, perf_domain_span(pd), sched_domain_span(sd)) {
    + if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
    + continue;
    +
    + /* Skip CPUs that will be overutilized. */
    + util = cpu_util_next(cpu, p, cpu);
    + cpu_cap = capacity_of(cpu);
    + if (cpu_cap * 1024 < util * capacity_margin)
    + continue;
    +
    + /* Always use prev_cpu as a candidate. */
    + if (cpu == prev_cpu) {
    + prev_energy = compute_energy(p, prev_cpu, head);
    + best_energy = min(best_energy, prev_energy);
    + continue;
    + }
    +
    + /*
    + * Find the CPU with the maximum spare capacity in
    + * the performance domain
    + */
    + spare_cap = cpu_cap - util;
    + if (spare_cap > max_spare_cap) {
    + max_spare_cap = spare_cap;
    + max_spare_cap_cpu = cpu;
    + }
    + }
    +
    + /* Evaluate the energy impact of using this CPU. */
    + if (max_spare_cap_cpu >= 0) {
    + cur_energy = compute_energy(p, max_spare_cap_cpu, head);
    + if (cur_energy < best_energy) {
    + best_energy = cur_energy;
    + best_energy_cpu = max_spare_cap_cpu;
    + }
    + }
    + }
    +unlock:
    + rcu_read_unlock();
    +
    + /*
    + * Pick the best CPU if prev_cpu cannot be used, or if it saves at
    + * least 6% of the energy used by prev_cpu.
    + */
    + if (prev_energy == ULONG_MAX)
    + return best_energy_cpu;
    +
    + if ((prev_energy - best_energy) > (prev_energy >> 4))
    + return best_energy_cpu;
    +
    + return prev_cpu;
    +
    +fail:
    + rcu_read_unlock();
    +
    + return -1;
    +}
    +
    /*
    * select_task_rq_fair: Select target runqueue for the waking task in domains
    * that have the 'sd_flag' flag set. In practice, this is SD_BALANCE_WAKE,
    @@ -6476,8 +6607,16 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f

    if (sd_flag & SD_BALANCE_WAKE) {
    record_wakee(p);
    - want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu)
    - && cpumask_test_cpu(cpu, &p->cpus_allowed);
    +
    + if (static_branch_unlikely(&sched_energy_present)) {
    + new_cpu = find_energy_efficient_cpu(p, prev_cpu);
    + if (new_cpu >= 0)
    + return new_cpu;
    + new_cpu = prev_cpu;
    + }
    +
    + want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu) &&
    + cpumask_test_cpu(cpu, &p->cpus_allowed);
    }

    rcu_read_lock();
    --
    2.19.2
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