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

[Con Kolivas <kernel@kolivas.org>]

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.

> iso_refractory is wrong. Isn't calling it from
> timer.c::update_process_times() better?
> 
> And shouldn't real RT task also counted? If RT tasks use 40% cpu you can
> lockup the system as unprivileged user with SCHED_ISO because it doesn't
> reach the 70% cpu limit.

Ah yes. Good point. Will add that to the equation.

> 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.

> sysrq-n causes a reboot.

And that will need looking into.

Thanks very much for your comments!

Cheers,
Con