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Subject[RFC 01/11] 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 | 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;

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