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...@intel.com>
Cc: Ben Segall <bseg...@google.com>
Cc: Daniel Bristot de Oliveira <bris...@redhat.com>
Cc: Dietmar Eggemann <dietmar.eggem...@arm.com>
Cc: Mel Gorman <mgor...@suse.de>
Cc: Quentin Perret <qper...@google.com>
Cc: Rafael J. Wysocki <rafael.j.wyso...@intel.com>
Cc: Srinivas Pandruvada <srinivas.pandruv...@linux.intel.com>
Cc: Steven Rostedt <rost...@goodmis.org>
Cc: Tim Chen <tim.c.c...@linux.intel.com>
Reviewed-by: Joel Fernandes (Google) <j...@joelfernandes.org>
Reviewed-by: Len Brown <len.br...@intel.com>
Signed-off-by: Ricardo Neri <ricardo.neri-calde...@linux.intel.com>
---
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
