[RFC 01/11] sched/fair: Fix select_idle_cpu()s cost accounting

[Peter Zijlstra]

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) <pet...@infradead.org>
---
 kernel/sched/fair.c |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6368,11 +6368,11 @@ static inline int select_idle_smt(struct
  */
 static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int 
target)
 {
+       int cpu, loops = 0, nr = INT_MAX;
        struct sched_domain *this_sd;
        u64 avg_cost, avg_idle;
        u64 time, cost;
        s64 delta;
-       int cpu, nr = INT_MAX;
 
        this_sd = rcu_dereference(*this_cpu_ptr(&sd_llc));
        if (!this_sd)
@@ -6399,8 +6399,10 @@ static int select_idle_cpu(struct task_s
        time = local_clock();
 
        for_each_cpu_wrap(cpu, sched_domain_span(sd), target) {
-               if (!--nr)
-                       return -1;
+               if (loops++ >= nr) {
+                       cpu = -1;
+                       break;
+               }
                if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
                        continue;
                if (available_idle_cpu(cpu))
@@ -6408,6 +6410,7 @@ static int select_idle_cpu(struct task_s
        }
 
        time = local_clock() - time;
+       time = div_u64(time, loops);
        cost = this_sd->avg_scan_cost;
        delta = (s64)(time - cost) / 8;
        this_sd->avg_scan_cost += delta;