[PATCH 1/5] sched: limit cpu search in select_idle_cpu

From: subhra mazumdar
Date: Tue Jun 12 2018 - 13:48:42 EST


Put upper and lower limit on cpu search of select_idle_cpu. The lower limit
is amount of cpus in a core while upper limit is twice that. This ensures
for any architecture we will usually search beyond a core. The upper limit
also helps in keeping the search cost low and constant.

Signed-off-by: subhra mazumdar <subhra.mazumdar@xxxxxxxxxx>
---
kernel/sched/fair.c | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e497c05..9a6d28d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6372,7 +6372,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
u64 avg_cost, avg_idle;
u64 time, cost;
s64 delta;
- int cpu, nr = INT_MAX;
+ int cpu, limit, floor, nr = INT_MAX;

this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
if (!this_sd)
@@ -6389,11 +6389,18 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
return -1;

if (sched_feat(SIS_PROP)) {
+ floor = cpumask_weight(topology_sibling_cpumask(target));
+ if (floor < 2)
+ floor = 2;
+ limit = 2*floor;
u64 span_avg = sd->span_weight * avg_idle;
- if (span_avg > 4*avg_cost)
+ if (span_avg > floor*avg_cost) {
nr = div_u64(span_avg, avg_cost);
- else
- nr = 4;
+ if (nr > limit)
+ nr = limit;
+ } else {
+ nr = floor;
+ }
}

time = local_clock();
--
2.9.3