[PATCH 3/8] sched/fair: Tweak misfit-related capacity checks

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

During load-balance, groups classified as group_misfit_task are filtered
out if they do not pass

  group_smaller_max_cpu_capacity(<candidate group>, <local group>);

which itself employs fits_capacity() to compare the sgc->max_capacity of
both groups.

Due to the underlying margin, fits_capacity(X, 1024) will return false for
any X > 819. Tough luck, the capacity_orig's on e.g. the Pixel 4 are
{261, 871, 1024}. If a CPU-bound task ends up on one of those "medium"
CPUs, misfit migration will never intentionally upmigrate it to a CPU of
higher capacity due to the aforementioned margin.

One may argue the 20% margin of fits_capacity() is excessive in the advent
of counter-enhanced load tracking (APERF/MPERF, AMUs), but one point here
is that fits_capacity() is meant to compare a utilization value to a
capacity value, whereas here it is being used to compare two capacity
values. As CPU capacity and task utilization have different dynamics, a
sensible approach here would be to add a new helper dedicated to comparing
CPU capacities.

Introduce capacity_greater(), which uses a 5% margin. Use it to replace the
existing capacity checks. Note that check_cpu_capacity() uses yet another
margin (sd->imbalance_pct), and is left alone for now.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
 kernel/sched/fair.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7d01fa0bfc7e..58ce0b22fcb0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -113,6 +113,13 @@ int __weak arch_asym_cpu_priority(int cpu)
  */
 #define fits_capacity(cap, max)	((cap) * 1280 < (max) * 1024)
 
+/*
+ * The margin used when comparing CPU capacities.
+ * is 'cap' noticeably greater than 'ref'
+ *
+ * (default: ~5%)
+ */
+#define capacity_greater(cap, ref) ((ref) * 1078 < (cap) * 1024)
 #endif
 
 #ifdef CONFIG_CFS_BANDWIDTH
@@ -8253,7 +8260,7 @@ check_cpu_capacity(struct rq *rq, struct sched_domain *sd)
 static inline int check_misfit_status(struct rq *rq, struct sched_domain *sd)
 {
 	return rq->misfit_task_load &&
-		(rq->cpu_capacity_orig < rq->rd->max_cpu_capacity ||
+		(capacity_greater(rq->rd->max_cpu_capacity, rq->cpu_capacity_orig) ||
 		 check_cpu_capacity(rq, sd));
 }
 
@@ -8352,7 +8359,7 @@ group_is_overloaded(unsigned int imbalance_pct, struct sg_lb_stats *sgs)
 static inline bool
 group_smaller_min_cpu_capacity(struct sched_group *sg, struct sched_group *ref)
 {
-	return fits_capacity(sg->sgc->min_capacity, ref->sgc->min_capacity);
+	return capacity_greater(ref->sgc->min_capacity, sg->sgc->min_capacity);
 }
 
 /*
@@ -8362,7 +8369,7 @@ group_smaller_min_cpu_capacity(struct sched_group *sg, struct sched_group *ref)
 static inline bool
 group_smaller_max_cpu_capacity(struct sched_group *sg, struct sched_group *ref)
 {
-	return fits_capacity(sg->sgc->max_capacity, ref->sgc->max_capacity);
+	return capacity_greater(ref->sgc->max_capacity, sg->sgc->max_capacity);
 }
 
 static inline enum
@@ -9421,7 +9428,7 @@ static struct rq *find_busiest_queue(struct lb_env *env,
 		 * average load.
 		 */
 		if (sd_has_asym_cpucapacity(env->sd) &&
-		    capacity_of(env->dst_cpu) < capacity &&
+		    !capacity_greater(capacity_of(env->dst_cpu), capacity) &&
 		    nr_running == 1)
 			continue;
 
-- 
2.27.0