From: Vincent Donnefort <vincent.donnefort@xxxxxxx>
[ Upstream commit fecfcbc288e9f4923f40fd23ca78a6acdc7fdf6c ]
RT keeps track of the utilization on a per-rq basis with the structure
avg_rt. This utilization is updated during task_tick_rt(),
put_prev_task_rt() and set_next_task_rt(). However, when the current
running task changes its policy, set_next_task_rt() which would usually
take care of updating the utilization when the rq starts running RT tasks,
will not see a such change, leaving the avg_rt structure outdated. When
that very same task will be dequeued later, put_prev_task_rt() will then
update the utilization, based on a wrong last_update_time, leading to a
huge spike in the RT utilization signal.
The signal would eventually recover from this issue after few ms. Even if
no RT tasks are run, avg_rt is also updated in __update_blocked_others().
But as the CPU capacity depends partly on the avg_rt, this issue has
nonetheless a significant impact on the scheduler.
Fix this issue by ensuring a load update when a running task changes
its policy to RT.
Fixes: 371bf427 ("sched/rt: Add rt_rq utilization tracking")
Signed-off-by: Vincent Donnefort <vincent.donnefort@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Link: https://lore.kernel.org/r/1624271872-211872-2-git-send-email-vincent.donnefort@xxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
kernel/sched/rt.c | 17 ++++++++++++-----
1 file changed, 12 insertions(+), 5 deletions(-)
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 8f720b71d13d..e617287052d5 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -2331,13 +2331,20 @@ void __init init_sched_rt_class(void)
static void switched_to_rt(struct rq *rq, struct task_struct *p)
{
/*
- * If we are already running, then there's nothing
- * that needs to be done. But if we are not running
- * we may need to preempt the current running task.
- * If that current running task is also an RT task
+ * If we are running, update the avg_rt tracking, as the running time
+ * will now on be accounted into the latter.
+ */
+ if (task_current(rq, p)) {
+ update_rt_rq_load_avg(rq_clock_pelt(rq), rq, 0);
+ return;
+ }
+
+ /*
+ * If we are not running we may need to preempt the current
+ * running task. If that current running task is also an RT task
* then see if we can move to another run queue.
*/
- if (task_on_rq_queued(p) && rq->curr != p) {
+ if (task_on_rq_queued(p)) {
#ifdef CONFIG_SMP
if (p->nr_cpus_allowed > 1 && rq->rt.overloaded)
rt_queue_push_tasks(rq);
--
2.30.2
