[PATCH v4 4/6] sched/fair: Skip misfit load accounting when the destination CPU cannot help

[Ricardo Neri]

In domains with asymmetric capacity, identifying misfit load in a
scheduling group is not useful when the destination CPU cannot help (i.e.,
its capacity exceeds the group's maximum CPU capacity by less than ~5%). In
such cases, it also prevents load balance among clusters of equal capacity
when CONFIG_SCHED_CLUSTER is enabled. This happens because
update_sd_pick_busiest() skips candidate groups of type misfit_task if the
destination CPU has similar capacity.

Skipping misfit load accounting in this situation allows the group to be
classified as has_spare or fully_busy and lets load balancing proceed. Keep
marking scheduling groups as overloaded when misfit tasks are present. The
sg_overloaded flag propagates to the root domain and allows bigger CPUs in
it to help via newly idle balance.

Reviewed-by: Christian Loehle <christian.loehle@xxxxxxx>
Reviewed-by: Chen Yu <yu.c.chen@xxxxxxxxx>
Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
---
Changes in v4:
 * Added Reviewed-by tags from Vincent and Chen Yu. Thanks!

Changes in v3:
 * Added Reviewed-by tag from Christian. Thanks!

Changes in v2:
 * Moved the check of the destination CPU capacity inside the code block
   used for SD_ASYM_CPUCAPACITY. v1 inadvertently broke the mutual
   exclusion of the sched_reduced_capacity() path.
 * Keep marking the root domain as overloaded to allow bigger CPUs to
   help. (sashiko)
 * Fixed patch description to clarify that the capacity_greater() looks
   for differences of 5% or more. (Christian)
 * Reworded the patch description for clarity.
 * I did not include the Reviewed-by tag from Christian since the patch
   changed functionally.
---
 kernel/sched/fair.c | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a30ba02df688..77554d7410ff 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11850,12 +11850,25 @@ static inline void update_sg_lb_stats(struct lb_env *env,
 			continue;
 
 		if (sd_flags & SD_ASYM_CPUCAPACITY) {
-			/* Check for a misfit task on the cpu */
-			if (sgs->group_misfit_task_load < rq->misfit_task_load) {
-				sgs->group_misfit_task_load = rq->misfit_task_load;
-
+			if (rq->misfit_task_load) {
+				/*
+				 * Always mark the root domain overloaded so big
+				 * CPUs can pick up misfit tasks via newly idle
+				 * balance.
+				 */
 				if (balancing_at_rd)
 					*sg_overloaded = 1;
+
+				/*
+				 * Only account misfit load if @dst_cpu can
+				 * help; otherwise, the group may be classified
+				 * as misfit_task and update_sd_pick_busiest()
+				 * will skip it.
+				 */
+				if (capacity_greater(capacity_of(env->dst_cpu),
+						     group->sgc->max_capacity) &&
+				    (sgs->group_misfit_task_load < rq->misfit_task_load))
+					sgs->group_misfit_task_load = rq->misfit_task_load;
 			}
 		} else if (env->idle && sched_reduced_capacity(rq, env->sd)) {
 			/* Check for a task running on a CPU with reduced capacity */

-- 
2.43.0