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ANDROID: sched/fair: Don't balance misfits if it would overload local group

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When load balancing in a system with misfit tasks present, if we always
pull a misfit task to the local group this can lead to pulling a running
task from a smaller capacity CPUs to a bigger CPU which is busy. In this
situation, the pulled task is likely not to get a chance to run before
an idle balance on another small CPU pulls it back. This penalises the
pulled task as it is stopped for a short amount of time and then likely
relocated to a different CPU (since the original CPU just did a NEWLY_IDLE
balance and reset the periodic interval).

If we only do this unconditionally for NEWLY_IDLE balance, we can be
sure that any tasks and load which are present on the local group are
related to short-running tasks which we are happy to displace for a
longer running task in a system with misfit tasks present.

However, other balance types should only pull a task if we think
that the local group is underutilized - checking the number of tasks
gives us a conservative estimate here since if they were short tasks
we would have been doing NEWLY_IDLE balances instead.

Change-Id: I710add1ab1139482620b6addc8370ad194791beb
Signed-off-by: Chris Redpath <chris.redpath@arm.com>
Signed-off-by: Quentin Perret <quentin.perret@arm.com>
This commit is contained in:
Chris Redpath
2018-06-05 11:47:57 +01:00
committed by Quentin Perret
parent f351885fc7
commit a7455f8123
1 changed files with 12 additions and 2 deletions
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14
kernel/sched/fair.c
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@@ -8509,8 +8509,18 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
(sds->avg_load - local->avg_load) * local->group_capacity
) / SCHED_CAPACITY_SCALE;
/* Boost imbalance to allow misfit task to be balanced. */
if (busiest->group_type == group_misfit_task) {
/* Boost imbalance to allow misfit task to be balanced.
* Always do this if we are doing a NEWLY_IDLE balance
* on the assumption that any tasks we have must not be
* long-running (and hence we cannot rely upon load).
* However if we are not idle, we should assume the tasks
* we have are longer running and not override load-based
* calculations above unless we are sure that the local
* group is underutilized.
*/
if (busiest->group_type == group_misfit_task &&
(env->idle == CPU_NEWLY_IDLE ||
local->sum_nr_running < local->group_weight)) {
env->imbalance = max_t(long, env->imbalance,
busiest->group_misfit_task_load);
}