From: Uladzislau 2 Rezki <uladzislau2.re...@sonymobile.com>
A load balancer calculates imbalance factor for particular sched
domain and tries to steal up the prescribed amount of weighted load.
However, a small imbalance factor would sometimes prevent us from
stealing any tasks at all. When a CPU is newly idle, it should
steal first task which meets the migration criteria.
There is a slight improvement when it comes to frame drops
(in my case drops per/two seconds). Basically a test case is
left finger swipe on the display (21 times, duration is 2
seconds + 1 second sleep between iterations):
0 Framedrops: 7 5
1 Framedrops: 5 3
2 Framedrops: 8 5
3 Framedrops: 4 5
4 Framedrops: 3 3
5 Framedrops: 6 4
6 Framedrops: 3 2
7 Framedrops: 3 4
8 Framedrops: 5 3
9 Framedrops: 3 3
10 Framedrops: 7 4
11 Framedrops: 3 4
12 Framedrops: 3 3
13 Framedrops: 3 3
14 Framedrops: 3 5
15 Framedrops: 7 3
16 Framedrops: 5 3
17 Framedrops: 3 2
18 Framedrops: 5 3
19 Framedrops: 4 3
20 Framedrops: 3 2
max is 8 vs 5; min is 3 vs 2.
As for applied load, it is not significant and is "light".
Signed-off-by: Uladzislau 2 Rezki <uladzislau2.re...@sonymobile.com>
---
kernel/sched/fair.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6559d19..b56b0c5 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6802,6 +6802,15 @@ static int detach_tasks(struct lb_env *env)
if (env->idle != CPU_NOT_IDLE && env->src_rq->nr_running <= 1)
break;
+ /*
+ * Another CPU can place tasks, since we do not hold dst_rq lock
+ * while doing balancing. If newly idle CPU already got
something,
+ * give up to reduce latency for CONFIG_PREEMPT kernels.
+ */
+ if (IS_ENABLED(CONFIG_PREEMPT) && env->idle == CPU_NEWLY_IDLE &&
+ env->dst_rq->nr_running > 0)
+ break;
+
p = list_first_entry(tasks, struct task_struct, se.group_node);
env->loop++;
@@ -6824,7 +6833,8 @@ static int detach_tasks(struct lb_env *env)
if (sched_feat(LB_MIN) && load < 16 &&
!env->sd->nr_balance_failed)
goto next;
- if ((load / 2) > env->imbalance)
+ if ((!IS_ENABLED(CONFIG_PREEMPT) || env->idle !=
CPU_NEWLY_IDLE) &&
+ (load / 2) > env->imbalance)
goto next;
detach_task(p, env);
--
2.1.4
