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

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Message-ID: <20150107080155.1d42d123@luca-1225C>
Date:	Wed, 7 Jan 2015 08:01:55 +0100
From:	Luca Abeni <luca.abeni@...tn.it>
To:	tkhai@...dex.ru
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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 Tue, 06 Jan 2015 02:07:21 +0300
Kirill Tkhai <tkhai@...dex.ru> wrote:

> On Пн, 2015-01-05 at 16:21 +0100, Luca Abeni wrote:
[...]
> > 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().
> 
> It seems this way has problems. The first one is that task may become
> throttled again, and we will start dl_timer again.
Well, in my understanding if I change the parameters of a
SCHED_DEADLINE task when it is throttled, it stays throttled... So, the
task might not become throttled again before the dl timer fires.
So, I hoped this problem does not exist. But I might be wrong.

> The second is that
> it's better to minimize number of combination of situations we have.
> Let's keep only one combination: timer is set <-> task is throttled.
Yes, this was my goal too... So, if I change the parameters of a task
when it is throttled, I leave dl_throttled set to 1 and I leave the
timer active.

[...]
> > > @@ -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?
> 
> I think that when people change task's parameters, they want the
> kernel reacts on this immediately. For example, you want to kill
> throttled deadline task. You change parameters, but nothing happens.
> I think all developers had this use case when they were debugging
> deadline class.
I see... Different people have different requirements :)
My goal was to do something like adaptive scheduling (or scheduling
tasks with mode changes), so I did not want that changing the
scheduling parameters of a task affected the scheduling of the other
tasks... But if a task exits the throttled state when I change its
parameters, it might consume much more than the reserved CPU time.
Also, I suspect this kind of approach can be exploited by malicious
users: if I create a task with runtime 30ms and period 100ms, and I
change its scheduling parameters (to runtime=29ms and back) frequently
enough, I can consume much more than 30% of the CPU time...

Anyway, I am fine with every patch that fixes the bug :)



			Thanks,
				Luca
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