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Subject[PATCH v3 10/10] sched/fair: optimize find_idlest_group
Date
find_idlest_group() now loads CPU's load_avg in 2 different ways.
Consolidate the function to read and use load_avg only once and simplify
the algorithm to only look for the group with lowest load_avg.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
kernel/sched/fair.c | 52 +++++++++++-----------------------------------------
1 file changed, 11 insertions(+), 41 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 39a37ae..1fac444 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5560,16 +5560,14 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
{
struct sched_group *idlest = NULL, *group = sd->groups;
struct sched_group *most_spare_sg = NULL;
- unsigned long min_runnable_load = ULONG_MAX;
- unsigned long this_runnable_load = ULONG_MAX;
- unsigned long min_avg_load = ULONG_MAX, this_avg_load = ULONG_MAX;
+ unsigned long min_load = ULONG_MAX, this_load = ULONG_MAX;
unsigned long most_spare = 0, this_spare = 0;
int imbalance_scale = 100 + (sd->imbalance_pct-100)/2;
unsigned long imbalance = scale_load_down(NICE_0_LOAD) *
(sd->imbalance_pct-100) / 100;

do {
- unsigned long load, avg_load, runnable_load;
+ unsigned long load;
unsigned long spare_cap, max_spare_cap;
int local_group;
int i;
@@ -5586,15 +5584,11 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
* Tally up the load of all CPUs in the group and find
* the group containing the CPU with most spare capacity.
*/
- avg_load = 0;
- runnable_load = 0;
+ load = 0;
max_spare_cap = 0;

for_each_cpu(i, sched_group_span(group)) {
- load = cpu_load(cpu_rq(i));
- runnable_load += load;
-
- avg_load += cfs_rq_load_avg(&cpu_rq(i)->cfs);
+ load += cpu_load(cpu_rq(i));

spare_cap = capacity_spare_without(i, p);

@@ -5603,31 +5597,15 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
}

/* Adjust by relative CPU capacity of the group */
- avg_load = (avg_load * SCHED_CAPACITY_SCALE) /
- group->sgc->capacity;
- runnable_load = (runnable_load * SCHED_CAPACITY_SCALE) /
+ load = (load * SCHED_CAPACITY_SCALE) /
group->sgc->capacity;

if (local_group) {
- this_runnable_load = runnable_load;
- this_avg_load = avg_load;
+ this_load = load;
this_spare = max_spare_cap;
} else {
- if (min_runnable_load > (runnable_load + imbalance)) {
- /*
- * The runnable load is significantly smaller
- * so we can pick this new CPU:
- */
- min_runnable_load = runnable_load;
- min_avg_load = avg_load;
- idlest = group;
- } else if ((runnable_load < (min_runnable_load + imbalance)) &&
- (100*min_avg_load > imbalance_scale*avg_load)) {
- /*
- * The runnable loads are close so take the
- * blocked load into account through avg_load:
- */
- min_avg_load = avg_load;
+ if (load < min_load) {
+ min_load = load;
idlest = group;
}

@@ -5668,18 +5646,10 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
* local domain to be very lightly loaded relative to the remote
* domains but "imbalance" skews the comparison making remote CPUs
* look much more favourable. When considering cross-domain, add
- * imbalance to the runnable load on the remote node and consider
- * staying local.
+ * imbalance to the load on the remote node and consider staying
+ * local.
*/
- if ((sd->flags & SD_NUMA) &&
- min_runnable_load + imbalance >= this_runnable_load)
- return NULL;
-
- if (min_runnable_load > (this_runnable_load + imbalance))
- return NULL;
-
- if ((this_runnable_load < (min_runnable_load + imbalance)) &&
- (100*this_avg_load < imbalance_scale*min_avg_load))
+ if (min_load + imbalance >= this_load)
return NULL;

return idlest;
--
2.7.4
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