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 <r...@surriel.com>
Reviewed-by: Qais Yousef <qais.you...@arm.com>
Signed-off-by: Valentin Schneider <valentin.schnei...@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 f50a902bdf24..db892f6e222f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6300,15 +6300,8 @@ 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;
@@ -8467,7 +8460,7 @@ 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;
@@ -8524,7 +8517,8 @@ 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;
@@ -8607,7 +8601,7 @@ 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;
@@ -8730,7 +8724,7 @@ 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;
}
@@ -9408,7 +9402,7 @@ 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;
@@ -10225,6 +10219,7 @@ 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 d2e09a647c4f..27bf70bc86c7 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1492,6 +1492,39 @@ 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;
/*
--
2.25.1
