[PATCH 1/5] sched/fair: Remove SIS_AVG_CPU

Mel Gorman
2021-01-19T11:39:49Z

SIS_AVG_CPU was introduced as a means of avoiding a search when the
average search cost indicated that the search would likely fail. It was
a blunt instrument and disabled by commit 4c77b18cf8b7 ("sched/fair: Make
select_idle_cpu() more aggressive") and later replaced with a proportional
search depth by commit 1ad3aaf3fcd2 ("sched/core: Implement new approach
to scale select_idle_cpu()").

While there are corner cases where SIS_AVG_CPU is better, it has now been
disabled for almost three years. As the intent of SIS_PROP is to reduce
the time complexity of select_idle_cpu(), lets drop SIS_AVG_CPU and focus
on SIS_PROP as a throttling mechanism.

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
---
kernel/sched/fair.c | 20 +++++++++-----------
kernel/sched/features.h | 1 -
2 files changed, 9 insertions(+), 12 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 04a3ce20da67..9f5682aeda2e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6145,7 +6145,6 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
{
struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
struct sched_domain *this_sd;
- u64 avg_cost, avg_idle;
u64 time;
int this = smp_processor_id();
int cpu, nr = INT_MAX;
@@ -6154,18 +6153,17 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
if (!this_sd)
return -1;

- /*
- * Due to large variance we need a large fuzz factor; hackbench in
- * particularly is sensitive here.
- */
- avg_idle = this_rq()->avg_idle / 512;
- avg_cost = this_sd->avg_scan_cost + 1;
+ if (sched_feat(SIS_PROP)) {
+ u64 avg_cost, avg_idle, span_avg;

- if (sched_feat(SIS_AVG_CPU) && avg_idle < avg_cost)
- return -1;
+ /*
+ * Due to large variance we need a large fuzz factor;
+ * hackbench in particularly is sensitive here.
+ */
+ avg_idle = this_rq()->avg_idle / 512;
+ avg_cost = this_sd->avg_scan_cost + 1;

- if (sched_feat(SIS_PROP)) {
- u64 span_avg = sd->span_weight * avg_idle;
+ span_avg = sd->span_weight * avg_idle;
if (span_avg > 4*avg_cost)
nr = div_u64(span_avg, avg_cost);
else
diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index 68d369cba9e4..e875eabb6600 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -54,7 +54,6 @@ SCHED_FEAT(TTWU_QUEUE, true)
/*
* When doing wakeups, attempt to limit superfluous scans of the LLC domain.
*/
-SCHED_FEAT(SIS_AVG_CPU, false)
SCHED_FEAT(SIS_PROP, true)

/*
--
2.26.2