[PATCH v5 3/3] sched/fair: Introduce a CPU capacity comparison helper

[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.

Also note that comparing capacity extrema of local and source sched_group's
doesn't make much sense when at the day of the day the imbalance will be
pulled by a known env->dst_cpu, whose capacity can be anywhere within the
local group's capacity extrema.

While at it, replace group_smaller_{min, max}_cpu_capacity() with
comparisons of the source group's min/max capacity and the destination
CPU's capacity.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Reviewed-by: Qais Yousef <qais.yousef@arm.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>
---
 kernel/sched/fair.c | 33 ++++++++++-----------------------
 1 file changed, 10 insertions(+), 23 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d8077f82a380..c9c5c2697998 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 'cap1' noticeably greater than 'cap2'
+ *
+ * (default: ~5%)
+ */
+#define capacity_greater(cap1, cap2) ((cap1) * 1024 > (cap2) * 1078)
 #endif
 
 #ifdef CONFIG_CFS_BANDWIDTH
@@ -8364,26 +8371,6 @@ group_is_overloaded(unsigned int imbalance_pct, struct sg_lb_stats *sgs)
 	return false;
 }
 
-/*
- * group_smaller_min_cpu_capacity: Returns true if sched_group sg has smaller
- * per-CPU capacity than sched_group ref.
- */
-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);
-}
-
-/*
- * group_smaller_max_cpu_capacity: Returns true if sched_group sg has smaller
- * per-CPU capacity_orig than 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);
-}
-
 static inline enum
 group_type group_classify(unsigned int imbalance_pct,
 			  struct sched_group *group,
@@ -8539,7 +8526,7 @@ static bool update_sd_pick_busiest(struct lb_env *env,
 	 * internally or be covered by avg_load imbalance (eventually).
 	 */
 	if (sgs->group_type == group_misfit_task &&
-	    (!group_smaller_max_cpu_capacity(sg, sds->local) ||
+	    (!capacity_greater(capacity_of(env->dst_cpu), sg->sgc->max_capacity) ||
 	     sds->local_stat.group_type != group_has_spare))
 		return false;
 
@@ -8623,7 +8610,7 @@ static bool update_sd_pick_busiest(struct lb_env *env,
 	 */
 	if ((env->sd->flags & SD_ASYM_CPUCAPACITY) &&
 	    (sgs->group_type <= group_fully_busy) &&
-	    (group_smaller_min_cpu_capacity(sds->local, sg)))
+	    (capacity_greater(sg->sgc->min_capacity, capacity_of(env->dst_cpu))))
 		return false;
 
 	return true;
@@ -9423,7 +9410,7 @@ static struct rq *find_busiest_queue(struct lb_env *env,
 		 * average load.
 		 */
 		if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
-		    capacity_of(env->dst_cpu) < capacity &&
+		    !capacity_greater(capacity_of(env->dst_cpu), capacity) &&
 		    nr_running == 1)
 			continue;
 
-- 
2.25.1