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. Cleanup the LBF_DST_PINNED active balance special case.
Signed-off-by: Valentin Schneider <valentin.schnei...@arm.com>
Reviewed-by: Vincent Guittot <vincent.guit...@linaro.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggem...@arm.com>
---
kernel/sched/fair.c | 31 +++++++++++++++----------------
1 file changed, 15 insertions(+), 16 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 04d5e14fa261..d8077f82a380 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7422,6 +7422,7 @@ enum migration_type {
#define LBF_NEED_BREAK 0x02
#define LBF_DST_PINNED 0x04
#define LBF_SOME_PINNED 0x08
+#define LBF_ACTIVE_LB 0x10
struct lb_env {
struct sched_domain *sd;
@@ -7583,10 +7584,13 @@ int can_migrate_task(struct task_struct *p, struct
lb_env *env)
* our sched_group. We may want to revisit it if we couldn't
* meet load balance goals by pulling other tasks on src_cpu.
*
- * Avoid computing new_dst_cpu for NEWLY_IDLE or if we have
- * already computed one in current iteration.
+ * Avoid computing new_dst_cpu
+ * - for NEWLY_IDLE
+ * - if we have already computed one in current iteration
+ * - if it's an active balance
*/
- if (env->idle == CPU_NEWLY_IDLE || (env->flags &
LBF_DST_PINNED))
+ if (env->idle == CPU_NEWLY_IDLE ||
+ env->flags & (LBF_DST_PINNED | LBF_ACTIVE_LB))
return 0;
/* Prevent to re-select dst_cpu via env's CPUs: */
@@ -7611,10 +7615,14 @@ 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 +9813,6 @@ 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;
@@ -9957,13 +9962,7 @@ static int active_load_balance_cpu_stop(void *data)
.src_cpu = busiest_rq->cpu,
.src_rq = busiest_rq,
.idle = CPU_IDLE,
- /*
- * can_migrate_task() doesn't need to compute
new_dst_cpu
- * for active balancing. Since we have CPU_IDLE, but no
- * @dst_grpmask we need to make that test go away with
lying
- * about DST_PINNED.
- */
- .flags = LBF_DST_PINNED,
+ .flags = LBF_ACTIVE_LB,
};
schedstat_inc(sd->alb_count);
--
2.25.1
