[PATCH v4 04/12] sched/fair: Let low-priority cores help high-priority busy SMT cores

[Ricardo Neri]

Using asym_packing priorities within an SMT core is straightforward. Just
follow the priorities that hardware indicates.

When balancing load from an SMT core, also consider the idle state of its
siblings. Priorities do not reflect that an SMT core divides its throughput
among all its busy siblings. They only makes sense when exactly one sibling
is busy.

Indicate that active balance is needed if the destination CPU has lower
priority than the source CPU but the latter has busy SMT siblings.

Make find_busiest_queue() not skip higher-priority SMT cores with more than
busy sibling.

Cc: Ben Segall <bsegall@xxxxxxxxxx>
Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Ionela Voinescu <ionela.voinescu@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
Tested-by: Zhang Rui <rui.zhang@xxxxxxxxx>
Suggested-by: Valentin Schneider <vschneid@xxxxxxxxxx>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
---
Changes since v3:
 * Improved the logic to determine whether CPU priority should be followed.
   Also, wrapped this logic in a helper function. (Vincent G./ Peter)

Changes since v2:
 * Introduced this patch.

Changes since v1:
 * N/A
---
 kernel/sched/fair.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b6bbe0300635..b151e93ec316 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10488,8 +10488,15 @@ 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 */
+		/*
+		 * Make sure we only pull tasks from a CPU of lower priority
+		 * when balancing between SMT siblings.
+		 *
+		 * If balancing between cores, let lower priority CPUs help
+		 * SMT cores with more than one busy sibling.
+		 */
 		if ((env->sd->flags & SD_ASYM_PACKING) &&
+		    sched_use_asym_prio(env->sd, i) &&
 		    sched_asym_prefer(i, env->dst_cpu) &&
 		    nr_running == 1)
 			continue;
@@ -10582,10 +10589,15 @@ asym_active_balance(struct lb_env *env)
 	 * priority CPUs in order to pack all tasks in the highest priority
 	 * CPUs. When done between cores, do it only if the whole core if the
 	 * whole core is idle.
+	 *
+	 * If @env::src_cpu is an SMT core with busy siblings, let
+	 * the lower priority @env::dst_cpu help it. Do not follow
+	 * CPU priority.
 	 */
 	return env->idle != CPU_NOT_IDLE && (env->sd->flags & SD_ASYM_PACKING) &&
 	       sched_use_asym_prio(env->sd, env->dst_cpu) &&
-	       sched_asym_prefer(env->dst_cpu, env->src_cpu);
+	       (sched_asym_prefer(env->dst_cpu, env->src_cpu) ||
+		!sched_use_asym_prio(env->sd, env->src_cpu));
 }
 
 static inline bool
-- 
2.25.1