sched/fair: Consider SMT in ASYM_PACKING load balance

When deciding to pull tasks in ASYM_PACKING, it is necessary not only to
check for the idle state of the destination CPU, dst_cpu, but also of
its SMT siblings.

If dst_cpu is idle but its SMT siblings are busy, performance suffers
if it pulls tasks from a medium priority CPU that does not have SMT
siblings.

Implement asym_smt_can_pull_tasks() to inspect the state of the SMT
siblings of both dst_cpu and the CPUs in the candidate busiest group.

Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Reviewed-by: Len Brown <len.brown@intel.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/20210911011819.12184-7-ricardo.neri-calderon@linux.intel.com

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1c8b5fa..c50ae23 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8571,10 +8571,96 @@ group_type group_classify(unsigned int imbalance_pct,
        return group_has_spare;
 }
 
+/**
+ * asym_smt_can_pull_tasks - Check whether the load balancing CPU can pull tasks
+ * @dst_cpu:   Destination CPU of the load balancing
+ * @sds:       Load-balancing data with statistics of the local group
+ * @sgs:       Load-balancing statistics of the candidate busiest group
+ * @sg:                The candidate busiest group
+ *
+ * Check the state of the SMT siblings of both @sds::local and @sg and decide
+ * if @dst_cpu can pull tasks.
+ *
+ * If @dst_cpu does not have SMT siblings, it can pull tasks if two or more of
+ * the SMT siblings of @sg are busy. If only one CPU in @sg is busy, pull tasks
+ * only if @dst_cpu has higher priority.
+ *
+ * If both @dst_cpu and @sg have SMT siblings, and @sg has exactly one more
+ * busy CPU than @sds::local, let @dst_cpu pull tasks if it has higher priority.
+ * Bigger imbalances in the number of busy CPUs will be dealt with in
+ * update_sd_pick_busiest().
+ *
+ * If @sg does not have SMT siblings, only pull tasks if all of the SMT siblings
+ * of @dst_cpu are idle and @sg has lower priority.
+ */
+static bool asym_smt_can_pull_tasks(int dst_cpu, struct sd_lb_stats *sds,
+                                   struct sg_lb_stats *sgs,
+                                   struct sched_group *sg)
+{
+#ifdef CONFIG_SCHED_SMT
+       bool local_is_smt, sg_is_smt;
+       int sg_busy_cpus;
+
+       local_is_smt = sds->local->flags & SD_SHARE_CPUCAPACITY;
+       sg_is_smt = sg->flags & SD_SHARE_CPUCAPACITY;
+
+       sg_busy_cpus = sgs->group_weight - sgs->idle_cpus;
+
+       if (!local_is_smt) {
+               /*
+                * If we are here, @dst_cpu is idle and does not have SMT
+                * siblings. Pull tasks if candidate group has two or more
+                * busy CPUs.
+                */
+               if (sg_busy_cpus >= 2) /* implies sg_is_smt */
+                       return true;
+
+               /*
+                * @dst_cpu does not have SMT siblings. @sg may have SMT
+                * siblings and only one is busy. In such case, @dst_cpu
+                * can help if it has higher priority and is idle (i.e.,
+                * it has no running tasks).
+                */
+               return sched_asym_prefer(dst_cpu, sg->asym_prefer_cpu);
+       }
+
+       /* @dst_cpu has SMT siblings. */
+
+       if (sg_is_smt) {
+               int local_busy_cpus = sds->local->group_weight -
+                                     sds->local_stat.idle_cpus;
+               int busy_cpus_delta = sg_busy_cpus - local_busy_cpus;
+
+               if (busy_cpus_delta == 1)
+                       return sched_asym_prefer(dst_cpu, sg->asym_prefer_cpu);
+
+               return false;
+       }
+
+       /*
+        * @sg does not have SMT siblings. Ensure that @sds::local does not end
+        * up with more than one busy SMT sibling and only pull tasks if there
+        * are not busy CPUs (i.e., no CPU has running tasks).
+        */
+       if (!sds->local_stat.sum_nr_running)
+               return sched_asym_prefer(dst_cpu, sg->asym_prefer_cpu);
+
+       return false;
+#else
+       /* Always return false so that callers deal with non-SMT cases. */
+       return false;
+#endif
+}
+
 static inline bool
 sched_asym(struct lb_env *env, struct sd_lb_stats *sds,  struct sg_lb_stats *sgs,
           struct sched_group *group)
 {
+       /* Only do SMT checks if either local or candidate have SMT siblings */
+       if ((sds->local->flags & SD_SHARE_CPUCAPACITY) ||
+           (group->flags & SD_SHARE_CPUCAPACITY))
+               return asym_smt_can_pull_tasks(env->dst_cpu, sds, sgs, group);
+
        return sched_asym_prefer(env->dst_cpu, group->asym_prefer_cpu);
 }
 
@@ -9580,6 +9666,12 @@ static struct rq *find_busiest_queue(struct lb_env *env,
                    nr_running == 1)
                        continue;
 
+               /* Make sure we only pull tasks from a CPU of lower priority */
+               if ((env->sd->flags & SD_ASYM_PACKING) &&
+                   sched_asym_prefer(i, env->dst_cpu) &&
+                   nr_running == 1)
+                       continue;
+
                switch (env->migration_type) {
                case migrate_load:
                        /*