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Subject[RFC][PATCH 1/5] sched/fair: Fix select_idle_cpu()s cost accounting
We compute the average cost of the total scan, but then use it as a
per-cpu scan cost when computing the scan proportion. Fix this by
properly computing a per-cpu scan cost.

This also fixes a bug where we would terminate early (!--nr, case) and
not account that cost at all.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/sched/fair.c | 13 +++++++++----
1 file changed, 9 insertions(+), 4 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6144,10 +6144,10 @@ static inline int select_idle_smt(struct
static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int target)
{
struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
+ int cpu, loops = 1, nr = INT_MAX;
+ int this = smp_processor_id();
struct sched_domain *this_sd;
u64 time;
- int this = smp_processor_id();
- int cpu, nr = INT_MAX;

this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
if (!this_sd)
@@ -6175,14 +6175,19 @@ static int select_idle_cpu(struct task_s
}

for_each_cpu_wrap(cpu, cpus, target) {
- if (!--nr)
- return -1;
if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
break;
+
+ if (loops >= nr) {
+ cpu = -1;
+ break;
+ }
+ loops++;
}

if (sched_feat(SIS_PROP)) {
time = cpu_clock(this) - time;
+ time = div_u64(time, loops);
update_avg(&this_sd->avg_scan_cost, time);
}


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