[PATCH] sched/fair: Make sched-idle cpu selection consistent throughout

From: Viresh Kumar
Date: Thu Oct 24 2019 - 02:47:10 EST


There are instances where we keep searching for an idle CPU despite
having a sched-idle cpu already (in find_idlest_group_cpu(),
select_idle_smt() and select_idle_cpu() and then there are places where
we don't necessarily do that and return a sched-idle cpu as soon as we
find one (in select_idle_sibling()). This looks a bit inconsistent and
it may be worth having the same policy everywhere.

On the other hand, choosing a sched-idle cpu over a idle one shall be
beneficial from performance point of view as well, as we don't need to
get the cpu online from a deep idle state which is quite a time
consuming process and delays the scheduling of the newly wakeup task.

This patch tries to simplify code around sched-idle cpu selection and
make it consistent throughout.

FWIW, tests were done with the help of rt-app (8 SCHED_OTHER and 5
SCHED_IDLE tasks, not bound to any cpu) on ARM platform (octa-core), and
no significant difference in scheduling latency of SCHED_OTHER tasks was
found.

Signed-off-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
---
kernel/sched/fair.c | 34 ++++++++++++----------------------
1 file changed, 12 insertions(+), 22 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a81c36472822..bb367f48c1ef 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5545,7 +5545,7 @@ find_idlest_group_cpu(struct sched_group *group, struct task_struct *p, int this
unsigned int min_exit_latency = UINT_MAX;
u64 latest_idle_timestamp = 0;
int least_loaded_cpu = this_cpu;
- int shallowest_idle_cpu = -1, si_cpu = -1;
+ int shallowest_idle_cpu = -1;
int i;

/* Check if we have any choice: */
@@ -5554,6 +5554,9 @@ find_idlest_group_cpu(struct sched_group *group, struct task_struct *p, int this

/* Traverse only the allowed CPUs */
for_each_cpu_and(i, sched_group_span(group), p->cpus_ptr) {
+ if (sched_idle_cpu(i))
+ return i;
+
if (available_idle_cpu(i)) {
struct rq *rq = cpu_rq(i);
struct cpuidle_state *idle = idle_get_state(rq);
@@ -5576,12 +5579,7 @@ find_idlest_group_cpu(struct sched_group *group, struct task_struct *p, int this
latest_idle_timestamp = rq->idle_stamp;
shallowest_idle_cpu = i;
}
- } else if (shallowest_idle_cpu == -1 && si_cpu == -1) {
- if (sched_idle_cpu(i)) {
- si_cpu = i;
- continue;
- }
-
+ } else if (shallowest_idle_cpu == -1) {
load = cpu_load(cpu_rq(i));
if (load < min_load) {
min_load = load;
@@ -5590,11 +5588,7 @@ find_idlest_group_cpu(struct sched_group *group, struct task_struct *p, int this
}
}

- if (shallowest_idle_cpu != -1)
- return shallowest_idle_cpu;
- if (si_cpu != -1)
- return si_cpu;
- return least_loaded_cpu;
+ return shallowest_idle_cpu != -1 ? shallowest_idle_cpu : least_loaded_cpu;
}

static inline int find_idlest_cpu(struct sched_domain *sd, struct task_struct *p,
@@ -5747,7 +5741,7 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
*/
static int select_idle_smt(struct task_struct *p, int target)
{
- int cpu, si_cpu = -1;
+ int cpu;

if (!static_branch_likely(&sched_smt_present))
return -1;
@@ -5755,13 +5749,11 @@ static int select_idle_smt(struct task_struct *p, int target)
for_each_cpu(cpu, cpu_smt_mask(target)) {
if (!cpumask_test_cpu(cpu, p->cpus_ptr))
continue;
- if (available_idle_cpu(cpu))
+ if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
return cpu;
- if (si_cpu == -1 && sched_idle_cpu(cpu))
- si_cpu = cpu;
}

- return si_cpu;
+ return -1;
}

#else /* CONFIG_SCHED_SMT */
@@ -5790,7 +5782,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
u64 time, cost;
s64 delta;
int this = smp_processor_id();
- int cpu, nr = INT_MAX, si_cpu = -1;
+ int cpu, nr = INT_MAX;

this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
if (!this_sd)
@@ -5818,13 +5810,11 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t

for_each_cpu_wrap(cpu, sched_domain_span(sd), target) {
if (!--nr)
- return si_cpu;
+ return -1;
if (!cpumask_test_cpu(cpu, p->cpus_ptr))
continue;
- if (available_idle_cpu(cpu))
+ if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
break;
- if (si_cpu == -1 && sched_idle_cpu(cpu))
- si_cpu = cpu;
}

time = cpu_clock(this) - time;
--
2.21.0.rc0.269.g1a574e7a288b