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Re: [RFC][PATCH 3/8] sched/deadline/rtmutex: Dont miss the dl_runtime/dl_period update

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  Re: [RFC][PATCH 3/8] sched/deadline/rtmutex: Dont miss the  dl_runtime/dl_period update Steven Rostedt <rostedt@goodmis.org> - 2016-06-15 18:40 +0200
    Re: [RFC][PATCH 3/8] sched/deadline/rtmutex: Dont miss the  dl_runtime/dl_period update Peter Zijlstra <peterz@infradead.org> - 2016-06-15 20:00 +0200

#1423208 — Re: [RFC][PATCH 3/8] sched/deadline/rtmutex: Dont miss the dl_runtime/dl_period update

On Tue, 07 Jun 2016 21:56:38 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> From: Xunlei Pang <xlpang@redhat.com>
> 
> Currently dl tasks will actually return at the very beginning
> of rt_mutex_adjust_prio_chain() in !detect_deadlock cases:
> 
>     if (waiter->prio == task->prio) {
>         if (!detect_deadlock)
>             goto out_unlock_pi; // out here
>         else
>             requeue = false;
>     }
> 
> As the deadline value of blocked deadline tasks(waiters) without
> changing their sched_class(thus prio doesn't change) never changes,
> this seems reasonable, but it actually misses the chance of updating
> rt_mutex_waiter's "dl_runtime(period)_copy" if a waiter updates its
> deadline parameters(dl_runtime, dl_period) or boosted waiter changes
> to !deadline class.
> 
> Thus, force deadline task not out by adding the !dl_prio() condition.
> 
> [peterz: I should introduce more task state comparators like
> rt_mutex_waiter_less, all PI prio comparisons already have this DL
> exception, except this one]
> 
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Juri Lelli <juri.lelli@arm.com>
> Signed-off-by: Xunlei Pang <xlpang@redhat.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Link: http://lkml.kernel.org/r/1460633827-345-7-git-send-email-xlpang@redhat.com
> ---
>  kernel/locking/rtmutex.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -488,7 +488,7 @@ static int rt_mutex_adjust_prio_chain(st
>  	 * enabled we continue, but stop the requeueing in the chain
>  	 * walk.
>  	 */
> -	if (waiter->prio == task->prio) {
> +	if (waiter->prio == task->prio && !dl_task(task)) {

Isn't task the owner of the lock? What happens if the waiter is a
deadline task?

-- Steve

>  		if (!detect_deadlock)
>  			goto out_unlock_pi;
>  		else
> 

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#1423272

On Wed, Jun 15, 2016 at 12:30:07PM -0400, Steven Rostedt wrote:

> > +++ b/kernel/locking/rtmutex.c
> > @@ -488,7 +488,7 @@ static int rt_mutex_adjust_prio_chain(st
> >  	 * enabled we continue, but stop the requeueing in the chain
> >  	 * walk.
> >  	 */
> > -	if (waiter->prio == task->prio) {
> > +	if (waiter->prio == task->prio && !dl_task(task)) {
> 
> Isn't task the owner of the lock? 

No, task is blocked on something.

> What happens if the waiter is a
> deadline task?

So the test here is a shortcut to terminate the Pi chain adjust, it says
that if the waiter and task have the same priority, we're done. Further
adjustments will not make a difference.

The problem is that for deadline tasks, prio is a useless number, so
even if they match (all deadline tasks have prio -1) they might still
not actually match.

After the last patch I suppose we could do something like:

	waiter->prio == task->prio && waiter->deadline == task->dl.deadline

In any case, any condition that compares just two 'prio' values is per
definition broken if DL tasks are involved.

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