linux-kernel - Re: [PATCH 05/16] sched: SCHED_DEADLINE policy implementation.

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Date:	Tue, 15 May 2012 12:10:16 +0200
From:	Juri Lelli <juri.lelli@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	tglx@...utronix.de, mingo@...hat.com, rostedt@...dmis.org,
	cfriesen@...tel.com, oleg@...hat.com, fweisbec@...il.com,
	darren@...art.com, johan.eker@...csson.com, p.faure@...tech.ch,
	linux-kernel@...r.kernel.org, claudio@...dence.eu.com,
	michael@...rulasolutions.com, fchecconi@...il.com,
	tommaso.cucinotta@...up.it, nicola.manica@...i.unitn.it,
	luca.abeni@...tn.it, dhaval.giani@...il.com, hgu1972@...il.com,
	paulmck@...ux.vnet.ibm.com, raistlin@...ux.it,
	insop.song@...csson.com, liming.wang@...driver.com
Subject: Re: [PATCH 05/16] sched: SCHED_DEADLINE policy implementation.

On 04/23/2012 05:43 PM, Peter Zijlstra wrote:
> On Mon, 2012-04-23 at 17:39 +0200, Juri Lelli wrote:
>> On 04/23/2012 04:35 PM, Peter Zijlstra wrote:
>>> On Fri, 2012-04-06 at 09:14 +0200, Juri Lelli wrote:
>>>> +static 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;
>>>> +       }
>>>
>>> Same question I guess, how can it be active here? Also, just letting it
>>> run doesn't seem like the best way out..
>>>
>>
>> Probably s/hrtimer_try_to_cancel/hrtimer_cancel is better.
>
> Yeah, not sure you can do hrtimer_cancel() there though, you're holding
> ->pi_lock and rq->lock and have IRQs disabled. That sounds like asking
> for trouble.
>
> Anyway, if it can't happen, we don't have to fix it.. so lets answer
> that first ;-)

Even if I dropped the bits for allowing !root users, this critical point
still remains.
What if I leave this like it is and instead I do the following?

@@ -488,9 +488,10 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
         /*
          * We need to take care of a possible races here. In fact, the
          * task might have changed its scheduling policy to something
-        * different from SCHED_DEADLINE (through sched_setscheduler()).
+        * different from SCHED_DEADLINE or changed its reservation
+        * parameters (through sched_{setscheduler(),setscheduler2()}).
          */
-       if (!dl_task(p))
+       if (!dl_task(p) || dl_se->dl_new)
                 goto unlock;
  
         dl_se->dl_throttled = 0;

The idea is that hrtimer_try_to_cancel should fail only if the callback routine
is running. If, meanwhile, I set up new parameters, I can try to recognize this
situation through dl_new (set to 1 during __setparam_dl).

BTW, I'd have a new version ready (also rebased on the current tip/master). It
address all the comments excluding your gcc work-around, math128 and
nr_cpus_allowed shift (patches are ready but those changes not yet mainline,
right?). Anyway, do you think would be fine to post it?

Thanks and Regards,

- Juri
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