Re: [PATCH 1/2] sched/dl: Implement cancel_dl_timer() to use in switched_from_dl()

[Juri Lelli <juri.lelli@arm.com>]

Hi Kirill,

On 27/09/14 09:40, Kirill Tkhai wrote:
> From: Kirill Tkhai <ktkhai@parallels.com>
> 
> hrtimer_try_to_cancel() may bring a suprise, and even two its calls
> may fail.
> 
> So, let's implement 100% guaranteed way to cancel the timer and let's
> be sure we are safe even in very unlikely situations.
> 

Could you give more details about the unlikely situations in the
changelog? It would be nice to have some more information for future
reference.

Thanks,

- Juri

> We do not create any problem with rq unlocking, because it already
> may happed below in pull_dl_task(). No problem with deadline tasks
> balancing too.
> 
> Signed-off-by: Kirill Tkhai <ktkhai@parallels.com>
> ---
>  kernel/sched/deadline.c |   33 ++++++++++++++++++++++++++-------
>  1 file changed, 26 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index abfaf3d..63f8b4a 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -555,11 +555,6 @@ void init_dl_task_timer(struct sched_dl_entity *dl_se)
>  {
>  	struct hrtimer *timer = &dl_se->dl_timer;
>  
> -	if (hrtimer_active(timer)) {
> -		hrtimer_try_to_cancel(timer);
> -		return;
> -	}
> -
>  	hrtimer_init(timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
>  	timer->function = dl_task_timer;
>  }
> @@ -1567,10 +1562,34 @@ void init_sched_dl_class(void)
>  
>  #endif /* CONFIG_SMP */
>  
> +/*
> + *  Surely cancel task's dl_timer. May drop rq->lock.
> + */
> +static void cancel_dl_timer(struct rq *rq, struct task_struct *p)
> +{
> +	struct hrtimer *dl_timer = &p->dl.dl_timer;
> +
> +	/* Nobody will change task's class if pi_lock is held */
> +	lockdep_assert_held(&p->pi_lock);
> +
> +	if (hrtimer_active(dl_timer)) {
> +		int ret = hrtimer_try_to_cancel(dl_timer);
> +
> +		if (unlikely(ret == -1)) {
> +			/*
> +			 * Note, p may migrate OR new deadline tasks
> +			 * may appear in rq when we are unlocking it.
> +			 */
> +			raw_spin_unlock(&rq->lock);
> +			hrtimer_cancel(dl_timer);
> +			raw_spin_lock(&rq->lock);
> +		}
> +	}
> +}
> +
>  static void switched_from_dl(struct rq *rq, struct task_struct *p)
>  {
> -	if (hrtimer_active(&p->dl.dl_timer) && !dl_policy(p->policy))
> -		hrtimer_try_to_cancel(&p->dl.dl_timer);
> +	cancel_dl_timer(rq, p);
>  
>  	__dl_clear_params(p);
>  
> 
>