linux-kernel - Re: Another SCHED_DEADLINE bug (with bisection and possible fix)

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Date:	Mon, 05 Jan 2015 16:21:31 +0100
From:	Luca Abeni <luca.abeni@...tn.it>
To:	Kirill Tkhai <tkhai@...dex.ru>,
	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Another SCHED_DEADLINE bug (with bisection and possible fix)

Hi Kirill,

On 01/04/2015 11:51 PM, Kirill Tkhai wrote:
> Hi, Luca,
>
> I've just notived this.
[...]
> I think we should do something like below.
>
> hrtimer_init() is already called in sched_fork(), so we shouldn't do this
> twice. Also, we shouldn't reinit dl_throttled etc if timer is pending,
> and we should prevent a race here.
Thanks for the comments (I did not notice that hrtimer_init() was called twice)
and for the patch. I'll test it in the next days.

For reference, I attach the patch I am using locally (based on what I suggested
in my previous mail) and seems to work fine here.

Based on your comments, I suspect my patch can be further simplified by moving
the call to init_dl_task_timer() in __sched_fork().

[...]
> @@ -3250,16 +3251,19 @@ static void
>   __setparam_dl(struct task_struct *p, const struct sched_attr *attr)
>   {
>   	struct sched_dl_entity *dl_se = &p->dl;
> +        struct hrtimer *timer = &dl_se->dl_timer;
> +
> +	if (!hrtimer_active(timer) || hrtimer_try_to_cancel(timer) != -1) {
Just for the sake of curiosity, why trying to cancel the timer
("|| hrtimer_try_to_cancel(timer)") here? If it is active, cannot we leave it
active (without touching dl_throttled, dl_new and dl_yielded)?

I mean: if I try to change the parameters of a task when it is throttled, I'd like
it to stay throttled until the end of the reservation period... Or am I missing
something?


				Thanks,
					Luca

> +		dl_se->dl_throttled = 0;
> +		dl_se->dl_new = 1;
> +		dl_se->dl_yielded = 0;
> +	}
>
> -	init_dl_task_timer(dl_se);
>   	dl_se->dl_runtime = attr->sched_runtime;
>   	dl_se->dl_deadline = attr->sched_deadline;
>   	dl_se->dl_period = attr->sched_period ?: dl_se->dl_deadline;
>   	dl_se->flags = attr->sched_flags;
>   	dl_se->dl_bw = to_ratio(dl_se->dl_period, dl_se->dl_runtime);
> -	dl_se->dl_throttled = 0;
> -	dl_se->dl_new = 1;
> -	dl_se->dl_yielded = 0;
>   }
>
>   /*
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index e5db8c6..8649bd6 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -500,7 +500,7 @@ static int start_dl_timer(struct sched_dl_entity *dl_se, bool boosted)
>    * updating (and the queueing back to dl_rq) will be done by the
>    * next call to enqueue_task_dl().
>    */
> -static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
> +enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
>   {
>   	struct sched_dl_entity *dl_se = container_of(timer,
>   						     struct sched_dl_entity,
> @@ -559,14 +559,6 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
>   	return HRTIMER_NORESTART;
>   }
>
> -void init_dl_task_timer(struct sched_dl_entity *dl_se)
> -{
> -	struct hrtimer *timer = &dl_se->dl_timer;
> -
> -	hrtimer_init(timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
> -	timer->function = dl_task_timer;
> -}
> -
>   static
>   int dl_runtime_exceeded(struct rq *rq, struct sched_dl_entity *dl_se)
>   {
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 9a2a45c..ad3a2f0 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1257,7 +1257,6 @@ extern void init_rt_bandwidth(struct rt_bandwidth *rt_b, u64 period, u64 runtime
>
>   extern struct dl_bandwidth def_dl_bandwidth;
>   extern void init_dl_bandwidth(struct dl_bandwidth *dl_b, u64 period, u64 runtime);
> -extern void init_dl_task_timer(struct sched_dl_entity *dl_se);
>
>   unsigned long to_ratio(u64 period, u64 runtime);
>
>


View attachment "0003-Do-not-initialize-the-deadline-timer-if-it-is-alread.patch" of type "text/x-diff" (1544 bytes)