linux-kernel - Re: WARN ON at kernel/sched/deadline.c task_non

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Date:   Wed, 13 Mar 2019 15:49:48 +0100
From:   luca abeni <luca.abeni@...tannapisa.it>
To:     "chengjian (D)" <cj.chengjian@...wei.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Li Bin <huawei.libin@...wei.com>,
        "Xiexiuqi (Xie XiuQi)" <xiexiuqi@...wei.com>, <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>
Subject: Re: WARN ON at kernel/sched/deadline.c task_non_contending

Hi,

(I added Juri in cc)

On Tue, 12 Mar 2019 10:03:12 +0800
"chengjian (D)" <cj.chengjian@...wei.com> wrote:
[...]
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 31c050a0d0ce..d73cb033a06d 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -252,7 +252,6 @@ static void task_non_contending(struct
> task_struct *p) if (dl_entity_is_special(dl_se))
>                  return;
> 
> -       WARN_ON(hrtimer_active(&dl_se->inactive_timer));
>          WARN_ON(dl_se->dl_non_contending);
> 
>          zerolag_time = dl_se->deadline -
> @@ -287,7 +286,9 @@ static void task_non_contending(struct
> task_struct *p) }
> 
>          dl_se->dl_non_contending = 1;
> -       get_task_struct(p);
> +
> +       if (!hrtimer_active(&dl_se->inactive_timer));
> +               get_task_struct(p);
>          hrtimer_start(timer, ns_to_ktime(zerolag_time),
> HRTIMER_MODE_REL); }

After looking at the patch a little bit more and running some tests,
I suspect this solution might be racy:
when the timer is already active, (and hrtimer_start() fails), it
relies on its handler to decrease the running bw (by setting
dl_non_contending to 1)... But inactive_task_timer() might have
already checked dl_non_contending, finding it equal to 0 (so, it
ends up doing nothing and the running bw is not decreased).


So, I would prefer a different solution. I think this patch should work:

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 6a73e41a2016..43901fa3f269 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -252,7 +252,6 @@ static void task_non_contending(struct task_struct *p)
 	if (dl_entity_is_special(dl_se))
 		return;
 
-	WARN_ON(hrtimer_active(&dl_se->inactive_timer));
 	WARN_ON(dl_se->dl_non_contending);
 
 	zerolag_time = dl_se->deadline -
@@ -269,7 +268,7 @@ static void task_non_contending(struct task_struct *p)
 	 * If the "0-lag time" already passed, decrease the active
 	 * utilization now, instead of starting a timer
 	 */
-	if (zerolag_time < 0) {
+	if ((zerolag_time < 0) || hrtimer_active(&dl_se->inactive_timer)) {
 		if (dl_task(p))
 			sub_running_bw(dl_se, dl_rq);
 		if (!dl_task(p) || p->state == TASK_DEAD) {


The idea is that if the timer is active, we leave dl_non_contending set to
0 (so that the timer handler does nothing), and we immediately decrease the
running bw.
I think this is OK, because this situation can happen only if the task
blocks, wakes up while the timer handler is running, and then immediately
blocks again - while the timer handler is still running. So, the "zero lag
time" cannot be too much in the future.


			Thanks,
				Luca