[PATCH v2 3/7] sched/fair: Add more sched_asym_cpucapacity static branch checks

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

Rik noted a while back that a handful of

  sd->flags & SD_ASYM_CPUCAPACITY

& family in the CFS load-balancer code aren't guarded by the
sched_asym_cpucapacity static branch.

Turning those checks into NOPs for those who don't need it is fairly
straightforward, and hiding it in a helper doesn't change code size in all
but one spot. It also gives us a place to document the differences between
checking the static key and checking the SD flag.

Suggested-by: Rik van Riel <riel@surriel.com>
Reviewed-by: Qais Yousef <qais.yousef@arm.com>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
 kernel/sched/fair.c  | 21 ++++++++-------------
 kernel/sched/sched.h | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+), 13 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 535ebc31c9a8..24119f9ad191 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6288,15 +6288,8 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 	 * sd_asym_cpucapacity rather than sd_llc.
 	 */
 	if (static_branch_unlikely(&sched_asym_cpucapacity)) {
+		/* See sd_has_asym_cpucapacity() */
 		sd = rcu_dereference(per_cpu(sd_asym_cpucapacity, target));
-		/*
-		 * On an asymmetric CPU capacity system where an exclusive
-		 * cpuset defines a symmetric island (i.e. one unique
-		 * capacity_orig value through the cpuset), the key will be set
-		 * but the CPUs within that cpuset will not have a domain with
-		 * SD_ASYM_CPUCAPACITY. These should follow the usual symmetric
-		 * capacity path.
-		 */
 		if (sd) {
 			i = select_idle_capacity(p, sd, target);
 			return ((unsigned)i < nr_cpumask_bits) ? i : target;
@@ -8440,7 +8433,7 @@ static inline void update_sg_lb_stats(struct lb_env *env,
 			continue;
 
 		/* Check for a misfit task on the cpu */
-		if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
+		if (sd_has_asym_cpucapacity(env->sd) &&
 		    sgs->group_misfit_task_load < rq->misfit_task_load) {
 			sgs->group_misfit_task_load = rq->misfit_task_load;
 			*sg_status |= SG_OVERLOAD;
@@ -8497,7 +8490,8 @@ static bool update_sd_pick_busiest(struct lb_env *env,
 	 * CPUs in the group should either be possible to resolve
 	 * internally or be covered by avg_load imbalance (eventually).
 	 */
-	if (sgs->group_type == group_misfit_task &&
+	if (static_branch_unlikely(&sched_asym_cpucapacity) &&
+	    sgs->group_type == group_misfit_task &&
 	    (!group_smaller_max_cpu_capacity(sg, sds->local) ||
 	     sds->local_stat.group_type != group_has_spare))
 		return false;
@@ -8580,7 +8574,7 @@ static bool update_sd_pick_busiest(struct lb_env *env,
 	 * throughput. Maximize throughput, power/energy consequences are not
 	 * considered.
 	 */
-	if ((env->sd->flags & SD_ASYM_CPUCAPACITY) &&
+	if (sd_has_asym_cpucapacity(env->sd) &&
 	    (sgs->group_type <= group_fully_busy) &&
 	    (group_smaller_min_cpu_capacity(sds->local, sg)))
 		return false;
@@ -8703,7 +8697,7 @@ static inline void update_sg_wakeup_stats(struct sched_domain *sd,
 	}
 
 	/* Check if task fits in the group */
-	if (sd->flags & SD_ASYM_CPUCAPACITY &&
+	if (sd_has_asym_cpucapacity(sd) &&
 	    !task_fits_capacity(p, group->sgc->max_capacity)) {
 		sgs->group_misfit_task_load = 1;
 	}
@@ -9394,7 +9388,7 @@ static struct rq *find_busiest_queue(struct lb_env *env,
 		 * Higher per-CPU capacity is considered better than balancing
 		 * average load.
 		 */
-		if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
+		if (sd_has_asym_cpucapacity(env->sd) &&
 		    capacity_of(env->dst_cpu) < capacity &&
 		    nr_running == 1)
 			continue;
@@ -10224,6 +10218,7 @@ static void nohz_balancer_kick(struct rq *rq)
 		}
 	}
 
+	 /* See sd_has_asym_cpucapacity(). */
 	sd = rcu_dereference(per_cpu(sd_asym_cpucapacity, cpu));
 	if (sd) {
 		/*
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 10a1522b1e30..a447b3f28792 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1484,6 +1484,39 @@ DECLARE_PER_CPU(struct sched_domain __rcu *, sd_asym_packing);
 DECLARE_PER_CPU(struct sched_domain __rcu *, sd_asym_cpucapacity);
 extern struct static_key_false sched_asym_cpucapacity;
 
+/*
+ * Note that the static key is system-wide, but the visibility of
+ * SD_ASYM_CPUCAPACITY isn't. Thus the static key being enabled does not
+ * imply all CPUs can see asymmetry.
+ *
+ * Consider an asymmetric CPU capacity system such as:
+ *
+ * MC [           ]
+ *     0 1 2 3 4 5
+ *     L L L L B B
+ *
+ * w/ arch_scale_cpu_capacity(L) < arch_scale_cpu_capacity(B)
+ *
+ * By default, booting this system will enable the sched_asym_cpucapacity
+ * static key, and all CPUs will see SD_ASYM_CPUCAPACITY set at their MC
+ * sched_domain.
+ *
+ * Further consider exclusive cpusets creating a "symmetric island":
+ *
+ * MC [   ][      ]
+ *     0 1  2 3 4 5
+ *     L L  L L B B
+ *
+ * Again, booting this will enable the static key, but CPUs 0-1 will *not* have
+ * SD_ASYM_CPUCAPACITY set in any of their sched_domain. This is the intended
+ * behaviour, as CPUs 0-1 should be treated as a regular, isolated SMP system.
+ */
+static inline bool sd_has_asym_cpucapacity(struct sched_domain *sd)
+{
+	return static_branch_unlikely(&sched_asym_cpucapacity) &&
+		sd->flags & SD_ASYM_CPUCAPACITY;
+}
+
 struct sched_group_capacity {
 	atomic_t		ref;
 	/*
-- 
2.27.0