Re: [PATCH]sched: Isochronous class v2 for unprivileged softrt scheduling

[utz lehmann]

On Thu, 2005-01-20 at 11:33 +1100, Con Kolivas wrote:
> utz lehmann wrote:
> > @@ -2406,6 +2489,10 @@ void scheduler_tick(void)
> >  	task_t *p = current;
> >  
> >  	rq->timestamp_last_tick = sched_clock();
> > +	if (iso_task(p) && !rq->iso_refractory)
> > +		inc_iso_ticks(rq, p);
> > +	else 
> > +		dec_iso_ticks(rq, p);
> > 
> > scheduler_tick() is not only called by the timer interrupt but also form
> > the fork code. Is this intended? I think the accounting for
> 
> The calling from fork code only occurs if there is one millisecond of 
> time_slice left so it will only very rarely be hit. I dont think this 
> accounting problem is worth worrying about.

I had experimented with throttling runaway RT tasks. I use a similar
accounting. I saw a difference between counting with or without the
calling from fork. If i remember correctly the timeout expired too fast
if the non-RT load was "while /bin/true; do :; done".
With "while true; do :; done" ("true" is bash buildin) it worked good.
But maybe it's not important in the real world.

> 
> > Futher on i see a fundamental problem with this accounting for
> > iso_refractory. What if i manage as unprivileged user to run a SCHED_ISO
> > task which consumes all cpu and only sleeps very short during the timer
> > interrupt? I think this will nearly lockup or very slow down the system.
> > The iso_cpu limit can't guaranteed.
> 
> Right you are. The cpu accounting uses primitive on-interrupt run time 
> which as we know is not infallible. To extend this I'll have to keep a 
> timer based on the sched_clock which is already implemented. That's 
> something for me to work on.

If i understand sched_clock correctly it only has higher resolution if
you can use tsc. In the non tsc case it's jiffies based. (On x86).
I think you can easily fool a timer tick/jiffies based accounting and do
a local DoS.
Making SCHED_ISO privileged if you don't have a high resolution
sched_clock is ugly.
I really like the idea of a unprivileged SCHED_ISO but it has to be safe
for a multi user system. And the kernel default should be safe for multi
user.

cheers
utz

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