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sched/fair: discount task contribution to find CPU with lowest utilization

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In some cases, the new_util of a task can be the same on several
CPUs. This causes an issue because the target_util is only updated
if the current new_util is strictly smaller than target_util.

To fix that, the cpu_util_wake() return value is used alongside the
new_util value. If two CPUs compute the same new_util value,
we'll now also look at their cpu_util_wake() return value. In this
case, the CPU that last ran the task will be chosen in priority.

Change-Id: Ia1ea2c4b3ec39621372c2f748862317d5b497723
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
This commit is contained in:
Valentin Schneider 2017-03-03 11:43:03 +00:00 • committed by Amit Pundir
commit 619812e4cd
1 changed files with 13 additions and 4 deletions
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17
kernel/sched/fair.c
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@ -6221,7 +6221,8 @@ static inline int find_best_target(struct task_struct *p, bool boosted, bool pre
int i;
for_each_cpu_and(i, tsk_cpus_allowed(p), sched_group_cpus(sg)) {
unsigned long cur_capacity, new_util;
unsigned long cur_capacity, new_util, wake_util;
unsigned long min_wake_util = ULONG_MAX;
if (!cpu_online(i))
continue;
@ -6231,7 +6232,8 @@ static inline int find_best_target(struct task_struct *p, bool boosted, bool pre
* so prev_cpu will receive a negative bias due to the double
* accounting. However, the blocked utilization may be zero.
*/
new_util = cpu_util_wake(i, p) + task_util(p);
wake_util = cpu_util_wake(i, p);
new_util = wake_util + task_util(p);
/*
* Ensure minimum capacity to grant the required boost.
@ -6266,8 +6268,15 @@ static inline int find_best_target(struct task_struct *p, bool boosted, bool pre
* Find a target cpu with the lowest/highest
* utilization if prefer_idle/!prefer_idle.
*/
if ((prefer_idle && target_util > new_util) ||
(!prefer_idle && target_util < new_util)) {
if (prefer_idle) {
/* Favor the CPU that last ran the task */
if (new_util > target_util ||
wake_util > min_wake_util)
continue;
min_wake_util = wake_util;
target_util = new_util;
target_cpu = i;
} else if (target_util < new_util) {
target_util = new_util;
target_cpu = i;
}