On Sunday 22. March 2009 09.08.45 Chetan Nanda wrote:
> Hi All,
>
> I am experimenting with real time process on kernel version 2.6.24 (under
> UML) and having root privileges.
> I have written a simple code as follow:
>
> /*##############################*/
> int main()
> {
> pid_t pid;
> struct sched_param param;
> pid = getpid();
> memset((void *)¶m, 0 , sizeof(struct sched_param));
> param.sched_priority = 30;
>
> /*change the priority to rt*/
> sched_setscheduler(pid,SCHED_FIFO, ¶m);
>
> while(1);
very dangerous (and completely pointless). No rt-task should ever do this. If
you have a computational intensive task, you should not run that in rt-mode,
but as a high-priority userland task. If it also requires polling of some
devices at a very precise interval, you should split it. Things that
absolutely *must* run as rt-task, in one (or more) processes with rt-pri, the
rest as normal tasks. I cannot stress this enough :)
The kernel will freeze, and this is intentional - after all, you've just told
it to. The computer will continue to function though, because the absolute
highest priority is reserved for the kernel, so you can have a kernel thread
with slightly higher priority.
And you cannot disable interrupts.
But, everything else will freeze up. And I will repeat this: because *you*
asked it to do so. Running things on rt-level is great. But it places a lot of
responsibility on your shoulders. rt-priorities should come with the
spiderman-mantra: with great power, comes great responsibility :)
> }
> /*##############################*/
>
> On running this program cause system to hangs and this is as per the real
> time task implementation in the kernel.
> Which says a rt task with policy equal to 'SCHED_FIFO' will run till it
> voluntarily release CPU.
Or if a task with even higher priority becomes runnable.
> Is there any way to kill such task? Isn't this kind of behavior
> compromising system security?
Well.. If you have more than one core, you can do it via the shell (as the
shell will have much lower priority than 30 :) Then you can kill it like any
other task (given you have root-privileges.
If you are running this on a single-core setup, you better hope you have
another task running with higher priority that can kill the taskf or you once
it realizes the rt-tasks is hogging all the cpu.
> ~~
> Chetan Nanda
Henrik
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