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Subject[PATCH v10 5/7] sched/fair: Use the same cpumask per-PD throughout find_energy_efficient_cpu()
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From: Dietmar Eggemann <dietmar.eggemann@arm.com>

The Perf Domain (PD) cpumask (struct em_perf_domain.cpus) stays
invariant after Energy Model creation, i.e. it is not updated after
CPU hotplug operations.

That's why the PD mask is used in conjunction with the cpu_online_mask
(or Sched Domain cpumask). Thereby the cpu_online_mask is fetched
multiple times (in compute_energy()) during a run-queue selection
for a task.

cpu_online_mask may change during this time which can lead to wrong
energy calculations.

To be able to avoid this, use the select_rq_mask per-cpu cpumask to
create a cpumask out of PD cpumask and cpu_online_mask and pass it
through the function calls of the EAS run-queue selection path.

The PD cpumask for max_spare_cap_cpu/compute_prev_delta selection
(find_energy_efficient_cpu()) is now ANDed not only with the SD mask
but also with the cpu_online_mask. This is fine since this cpumask
has to be in syc with the one used for energy computation
(compute_energy()).
An exclusive cpuset setup with at least one asymmetric CPU capacity
island (hence the additional AND with the SD cpumask) is the obvious
exception here.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index bb5311cc5a44..08e604ddf520 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6701,14 +6701,14 @@ static unsigned long cpu_util_without(int cpu, struct task_struct *p)
* task.
*/
static long
-compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd)
+compute_energy(struct task_struct *p, int dst_cpu, struct cpumask *cpus,
+ struct perf_domain *pd)
{
- struct cpumask *pd_mask = perf_domain_span(pd);
unsigned long max_util = 0, sum_util = 0, cpu_cap;
int cpu;

- cpu_cap = arch_scale_cpu_capacity(cpumask_first(pd_mask));
- cpu_cap -= arch_scale_thermal_pressure(cpumask_first(pd_mask));
+ cpu_cap = arch_scale_cpu_capacity(cpumask_first(cpus));
+ cpu_cap -= arch_scale_thermal_pressure(cpumask_first(cpus));

/*
* The capacity state of CPUs of the current rd can be driven by CPUs
@@ -6719,7 +6719,7 @@ compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd)
* If an entire pd is outside of the current rd, it will not appear in
* its pd list and will not be accounted by compute_energy().
*/
- for_each_cpu_and(cpu, pd_mask, cpu_online_mask) {
+ for_each_cpu(cpu, cpus) {
unsigned long util_freq = cpu_util_next(cpu, p, dst_cpu);
unsigned long cpu_util, util_running = util_freq;
struct task_struct *tsk = NULL;
@@ -6806,6 +6806,7 @@ compute_energy(struct task_struct *p, int dst_cpu, struct perf_domain *pd)
*/
static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
{
+ struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_rq_mask);
unsigned long prev_delta = ULONG_MAX, best_delta = ULONG_MAX;
struct root_domain *rd = cpu_rq(smp_processor_id())->rd;
int cpu, best_energy_cpu = prev_cpu, target = -1;
@@ -6840,7 +6841,9 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
unsigned long base_energy_pd;
int max_spare_cap_cpu = -1;

- for_each_cpu_and(cpu, perf_domain_span(pd), sched_domain_span(sd)) {
+ cpumask_and(cpus, perf_domain_span(pd), cpu_online_mask);
+
+ for_each_cpu_and(cpu, cpus, sched_domain_span(sd)) {
if (!cpumask_test_cpu(cpu, p->cpus_ptr))
continue;

@@ -6877,12 +6880,12 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
continue;

/* Compute the 'base' energy of the pd, without @p */
- base_energy_pd = compute_energy(p, -1, pd);
+ base_energy_pd = compute_energy(p, -1, cpus, pd);
base_energy += base_energy_pd;

/* Evaluate the energy impact of using prev_cpu. */
if (compute_prev_delta) {
- prev_delta = compute_energy(p, prev_cpu, pd);
+ prev_delta = compute_energy(p, prev_cpu, cpus, pd);
if (prev_delta < base_energy_pd)
goto unlock;
prev_delta -= base_energy_pd;
@@ -6891,7 +6894,8 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)

/* Evaluate the energy impact of using max_spare_cap_cpu. */
if (max_spare_cap_cpu >= 0) {
- cur_delta = compute_energy(p, max_spare_cap_cpu, pd);
+ cur_delta = compute_energy(p, max_spare_cap_cpu, cpus,
+ pd);
if (cur_delta < base_energy_pd)
goto unlock;
cur_delta -= base_energy_pd;
--
2.36.1.255.ge46751e96f-goog
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