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Subject[tip: sched/core] sched/fair: Make sched-idle CPU selection consistent throughout
The following commit has been merged into the sched/core branch of tip:

Commit-ID: 17346452b25b98acfb395d2a82ec2e4ad0cb7a01
Gitweb: https://git.kernel.org/tip/17346452b25b98acfb395d2a82ec2e4ad0cb7a01
Author: Viresh Kumar <viresh.kumar@linaro.org>
AuthorDate: Thu, 14 Nov 2019 16:19:27 +05:30
Committer: Ingo Molnar <mingo@kernel.org>
CommitterDate: Wed, 25 Dec 2019 10:42:07 +01:00

sched/fair: Make sched-idle CPU selection consistent throughout

There are instances where we keep searching for an idle CPU despite
already having a sched-idle CPU (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 and power point of view as well, as we don't
need to get the CPU online from a deep idle state which wastes quite a
lot of time and energy and delays the scheduling of the newly woken up
task.

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

Testing is done with the help of rt-app on hikey board (ARM64 octa-core,
2 clusters, 0-3 and 4-7). The cpufreq governor was set to performance to
avoid any side affects from CPU frequency. Following are the tests
performed:

Test 1: 1-cfs-task:

A single SCHED_NORMAL task is pinned to CPU5 which runs for 2333 us
out of 7777 us (so gives time for the cluster to go in deep idle
state).

Test 2: 1-cfs-1-idle-task:

A single SCHED_NORMAL task is pinned on CPU5 and single SCHED_IDLE
task is pinned on CPU6 (to make sure cluster 1 doesn't go in deep idle
state).

Test 3: 1-cfs-8-idle-task:

A single SCHED_NORMAL task is pinned on CPU5 and eight SCHED_IDLE
tasks are created which run forever (not pinned anywhere, so they run
on all CPUs). Checked with kernelshark that as soon as NORMAL task
sleeps, the SCHED_IDLE task starts running on CPU5.

And here are the results on mean latency (in us), using the "st" tool.

$ st 1-cfs-task/rt-app-cfs_thread-0.log
N min max sum mean stddev
642 90 592 197180 307.134 109.906

$ st 1-cfs-1-idle-task/rt-app-cfs_thread-0.log
N min max sum mean stddev
642 67 311 113850 177.336 41.4251

$ st 1-cfs-8-idle-task/rt-app-cfs_thread-0.log
N min max sum mean stddev
643 29 173 41364 64.3297 13.2344

The mean latency when we need to:

- wakeup from deep idle state is 307 us.
- wakeup from shallow idle state is 177 us.
- preempt a SCHED_IDLE task is 64 us.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/b90cbcce608cef4e02a7bbfe178335f76d201bab.1573728344.git.viresh.kumar@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
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 8da0222..1f34fa9 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5588,7 +5588,7 @@ 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: */
@@ -5597,6 +5597,9 @@ 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);
@@ -5619,12 +5622,7 @@ 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;
@@ -5633,11 +5631,7 @@ 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,
@@ -5790,7 +5784,7 @@ 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;
@@ -5798,13 +5792,11 @@ 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 */
@@ -5834,7 +5826,7 @@ 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)
@@ -5864,11 +5856,9 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t

for_each_cpu_wrap(cpu, cpus, target) {
if (!--nr)
- return si_cpu;
- if (available_idle_cpu(cpu))
+ return -1;
+ 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;
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