Commit 4ca7dd5b authored Jun 05, 2018 by Chris Redpath Committed by Gerrit - the friendly Code Review server Jul 31, 2018

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

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>
Git-commit: 37e94467
Git-repo: https://android.googlesource.com/kernel/common/

Signed-off-by: Puja Gupta <pujag@codeaurora.org>

parent 44b893b6

kernel/sched/fair.c

+12 −2

Original line number	Diff line number	Diff line
		@@ -10117,8 +10117,18 @@ 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);
		}