[PATCH 8/8] sched/fair: Relax task_hot() for misfit tasks

[Valentin Schneider <valentin.schneider@arm.com>]

Misfit tasks can and will be preempted by the stopper to migrate them over
to a higher-capacity CPU. However, when runnable but not current misfit
tasks are scanned by the load balancer (i.e. detach_tasks()), the
task_hot() ratelimiting logic may prevent us from enqueuing said task onto
a higher-capacity CPU.

Align detach_tasks() with the active-balance logic and let it pick a
cache-hot misfit task when the destination CPU can provide a capacity
uplift.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
 kernel/sched/fair.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index cba9f97d9beb..c2351b87824f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7484,6 +7484,17 @@ static int task_hot(struct task_struct *p, struct lb_env *env)
 	if (env->sd->flags & SD_SHARE_CPUCAPACITY)
 		return 0;
 
+	/*
+	 * On a (sane) asymmetric CPU capacity system, the increase in compute
+	 * capacity should offset any potential performance hit caused by a
+	 * migration.
+	 */
+	if (sd_has_asym_cpucapacity(env->sd) &&
+	    env->idle != CPU_NOT_IDLE &&
+	    !task_fits_capacity(p, capacity_of(env->src_cpu)) &&
+	    cpu_capacity_greater(env->dst_cpu, env->src_cpu))
+		return 0;
+
 	/*
 	 * Buddy candidates are cache hot:
 	 */
-- 
2.27.0