[PATCH 1/8] sched/fair: Clean up active balance nr_balance_failed trickery

[Valentin Schneider <valentin.schneider@arm.com>]

When triggering an active load balance, sd->nr_balance_failed is set to
such a value that any further can_migrate_task() using said sd will ignore
the output of task_hot().

This behaviour makes sense, as active load balance intentionally preempts a
rq's running task to migrate it right away, but this asynchronous write is
a bit shoddy, as the stopper thread might run active_load_balance_cpu_stop
before the sd->nr_balance_failed write either becomes visible to the
stopper's CPU or even happens on the CPU that appended the stopper work.

Add a struct lb_env flag to denote active balancing, and use it in
can_migrate_task(). Remove the sd->nr_balance_failed write that served the
same purpose.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
 kernel/sched/fair.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 197a51473e0c..0f6a4e58ce3c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7423,6 +7423,7 @@ enum migration_type {
 #define LBF_SOME_PINNED	0x08
 #define LBF_NOHZ_STATS	0x10
 #define LBF_NOHZ_AGAIN	0x20
+#define LBF_ACTIVE_LB	0x40
 
 struct lb_env {
 	struct sched_domain	*sd;
@@ -7608,10 +7609,14 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
 
 	/*
 	 * Aggressive migration if:
-	 * 1) destination numa is preferred
-	 * 2) task is cache cold, or
-	 * 3) too many balance attempts have failed.
+	 * 1) active balance
+	 * 2) destination numa is preferred
+	 * 3) task is cache cold, or
+	 * 4) too many balance attempts have failed.
 	 */
+	if (env->flags & LBF_ACTIVE_LB)
+		return 1;
+
 	tsk_cache_hot = migrate_degrades_locality(p, env);
 	if (tsk_cache_hot == -1)
 		tsk_cache_hot = task_hot(p, env);
@@ -9805,9 +9810,6 @@ static int load_balance(int this_cpu, struct rq *this_rq,
 					active_load_balance_cpu_stop, busiest,
 					&busiest->active_balance_work);
 			}
-
-			/* We've kicked active balancing, force task migration. */
-			sd->nr_balance_failed = sd->cache_nice_tries+1;
 		}
 	} else {
 		sd->nr_balance_failed = 0;
@@ -9963,7 +9965,8 @@ static int active_load_balance_cpu_stop(void *data)
 			 * @dst_grpmask we need to make that test go away with lying
 			 * about DST_PINNED.
 			 */
-			.flags		= LBF_DST_PINNED,
+			.flags		= LBF_DST_PINNED |
+					  LBF_ACTIVE_LB,
 		};
 
 		schedstat_inc(sd->alb_count);
-- 
2.27.0