[PATCH v2 1/7] sched/fair: Generalize asym_packing logic for SMT local sched group

[Ricardo Neri]

When balancing load between two physical cores, an idle destination CPU can
help another core only if all of its SMT siblings are also idle. Otherwise,
there is not increase in throughput. It does not matter whether the other
core is composed of SMT siblings.

Simply check if there are any tasks running on the local group and the
other core has exactly one busy CPU before proceeding. Let
find_busiest_group() handle the case of more than one busy CPU. This is
true for SMT2, SMT4, SMT8, etc.

Cc: Ben Segall <bsegall@xxxxxxxxxx>
Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Len Brown <len.brown@xxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
Cc: Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Tim C. Chen <tim.c.chen@xxxxxxxxx>
Cc: Valentin Schneider <vschneid@xxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Reviewed-by: Len Brown <len.brown@xxxxxxxxx>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
---
Changes since v1:
 * Reworded commit message and inline comments for clarity.
 * Stated that this changeset does not impact STM4 or SMT8.
---
 kernel/sched/fair.c | 29 +++++++++--------------------
 1 file changed, 9 insertions(+), 20 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e4a0b8bd941c..18c672ff39ef 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8900,12 +8900,10 @@ static bool asym_smt_can_pull_tasks(int dst_cpu, struct sd_lb_stats *sds,
 				    struct sched_group *sg)
 {
 #ifdef CONFIG_SCHED_SMT
-	bool local_is_smt, sg_is_smt;
+	bool local_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) {
@@ -8926,25 +8924,16 @@ static bool asym_smt_can_pull_tasks(int dst_cpu, struct sd_lb_stats *sds,
 		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).
+	 * @dst_cpu has SMT siblings. Do asym_packing load balancing only if
+	 * all its siblings are idle (moving tasks between physical cores in
+	 * which some SMT siblings are busy results in the same throughput).
+	 *
+	 * If the difference in the number of busy CPUs is two or more, let
+	 * find_busiest_group() take care of it. We only care if @sg has
+	 * exactly one busy CPU. This covers SMT and non-SMT sched groups.
 	 */
-	if (!sds->local_stat.sum_nr_running)
+	if (sg_busy_cpus == 1 && !sds->local_stat.sum_nr_running)
 		return sched_asym_prefer(dst_cpu, sg->asym_prefer_cpu);
 
 	return false;
-- 
2.25.1