[PATCH v4 6/6] sched/fair: Consider SMT in ASYM_PACKING load balance

[Ricardo Neri]

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.

Cc: Aubrey Li <aubrey.li@xxxxxxxxx>
Cc: Ben Segall <bsegall@xxxxxxxxxx>
Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Quentin Perret <qperret@xxxxxxxxxx>
Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
Cc: Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
Reviewed-by: Len Brown <len.brown@xxxxxxxxx>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
---
Changes since v3:
  * Removed the arch_asym_check_smt_siblings() hook. Discussions with the
    powerpc folks showed that this patch should not impact them. Also, more
    recent powerpc processor no longer use asym_packing. (PeterZ)
  * Removed unnecessary local variable in asym_can_pull_tasks(). (Dietmar)
  * Removed unnecessary check for local CPUs when the local group has zero
    utilization. (Joel)
  * Renamed asym_can_pull_tasks() as asym_smt_can_pull_tasks() to reflect
    the fact that it deals with SMT cases.
  * Made asym_smt_can_pull_tasks() return false for !CONFIG_SCHED_SMT so
    that callers can deal with non-SMT cases.

Changes since v2:
  * Reworded the commit message to reflect updates in code.
  * Corrected misrepresentation of dst_cpu as the CPU doing the load
    balancing. (PeterZ)
  * Removed call to arch_asym_check_smt_siblings() as it is now called in
    sched_asym().

Changes since v1:
  * Don't bailout in update_sd_pick_busiest() if dst_cpu cannot pull
    tasks. Instead, reclassify the candidate busiest group, as it
    may still be selected. (PeterZ)
  * Avoid an expensive and unnecessary call to cpumask_weight() when
    determining if a sched_group is comprised of SMT siblings.
    (PeterZ).
---
 kernel/sched/fair.c | 95 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 95 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index dd411cefb63f..8a1a2a43732c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8531,10 +8531,99 @@ 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 busiet 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, even the number of idle CPUs
+ * between @sds::local and @sg. Thus, pull tasks from @sg if the difference
+ * between the number of busy CPUs is 2 or more. If the difference is of 1,
+ * only pull if @dst_cpu has higher priority. 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_is_smt && sg_busy_cpus >= 2)
+			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.
+		 */
+		return !sds->local_stat.group_util &&
+		       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;
+
+		/* Local can always help to even the number busy CPUs. */
+		if (busy_cpus_delta >= 2)
+			return true;
+
+		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. As CPUs move in and out of idle state frequently,
+	 * also check the group utilization to smoother the decision.
+	 */
+	if (!sds->local_stat.group_util)
+		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);
 }
 
@@ -9540,6 +9629,12 @@ 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:
 			/*
-- 
2.17.1