sched/fair: Select an energy-efficient CPU on task wake-up

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.

Signed-off-by: Quentin Perret <quentin.perret@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: adharmap@codeaurora.org
Cc: chris.redpath@arm.com
Cc: currojerez@riseup.net
Cc: dietmar.eggemann@arm.com
Cc: edubezval@gmail.com
Cc: gregkh@linuxfoundation.org
Cc: javi.merino@kernel.org
Cc: joel@joelfernandes.org
Cc: juri.lelli@redhat.com
Cc: morten.rasmussen@arm.com
Cc: patrick.bellasi@arm.com
Cc: pkondeti@codeaurora.org
Cc: rjw@rjwysocki.net
Cc: skannan@codeaurora.org
Cc: smuckle@google.com
Cc: srinivas.pandruvada@linux.intel.com
Cc: thara.gopinath@linaro.org
Cc: tkjos@google.com
Cc: valentin.schneider@arm.com
Cc: vincent.guittot@linaro.org
Cc: viresh.kumar@linaro.org
Link: https://lkml.kernel.org/r/20181203095628.11858-15-quentin.perret@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 141 insertions, 2 deletions

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b3c94584d947..ca469646ebe1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6454,6 +6454,137 @@ compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd)
 }
 
 /*
+ * 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,
  * SD_BALANCE_FORK, or SD_BALANCE_EXEC.
@@ -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();