lkml.org 
[lkml]   [2021]   [Jun]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [PATCH 1/2] sched/rt: Fix RT utilization tracking during policy change
On Fri, 18 Jun 2021 at 15:32, Vincent Donnefort
<vincent.donnefort@arm.com> wrote:
>
> 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 will not see a such change, leaving the avg_rt structure outdated.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 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@arm.com>
>
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index a525447..5ac0f31 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -2341,10 +2341,8 @@ 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 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) {
> @@ -2355,6 +2353,13 @@ static void switched_to_rt(struct rq *rq, struct task_struct *p)
> if (p->prio < rq->curr->prio && cpu_online(cpu_of(rq)))
> resched_curr(rq);
> }
> +
> + /*
> + * 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))

task_current(rq,p) equals the "rq->curr != p" above. Having 2
different ways to get the same value in the function is a bit
confusing.

And call it once

Otherwise, the fix looks good to me

> + update_rt_rq_load_avg(rq_clock_pelt(rq), rq, 0);
> }
>
> /*
> --
> 2.7.4
>

\
 
 \ /
  Last update: 2021-06-21 10:39    [W:0.158 / U:0.696 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site