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SubjectRe: can I bring Linux down by running "renice -20 cpu_intensive_process"?
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Mike Galbraith <efault@gmx.de> writes:

> On Thu, 2006-03-16 at 22:51 +0000, Måns Rullgård wrote:
>> Bill Davidsen <davidsen@tmr.com> writes:
>>
>> > Måns Rullgård wrote:
>> >> Maybe extending sysrq+n to lower the priority of -20 tasks would be a
>> >> good idea.
>> >>
>> > If it runs before the keyboard thread it doesn't matter...
>>
>> Of course not, but that's not generally the case.
>>
>> > But why should this hang anything, when there should be enough i/o
>> > to get out of the user process. There's a good fix for this, don't
>> > give this guy root any more ;-)
>>
>> Ever heard of bugs? Anyone developing a program can make a mistake.
>> If the program runs with realtime scheduling a bug that makes it enter
>> an infinite loop (or do something else that hogs the CPU) can be
>> difficult to find since it rather efficiently locks you out.
>
> Given that someone has already determined that installing a safety valve
> for RT tasks was worth while, and given that there is practically no
> difference between a nice -20 and the lowest RT priority, seems to me
> that extending that safety valve to cover reniced tasks is the
> obviously-correct thing to do. I think you should submit a patch.

Something like this ought to do it (untested):

--- kernel/sched.c.orig 2006-02-09 23:41:57.000000000 +0000
+++ kernel/sched.c 2006-03-17 22:16:46.257298014 +0000
@@ -5681,21 +5681,22 @@

read_lock_irq(&tasklist_lock);
for_each_process (p) {
- if (!rt_task(p))
- continue;
+ if (rt_task(p)) {
+ rq = task_rq_lock(p, &flags);

- rq = task_rq_lock(p, &flags);
-
- array = p->array;
- if (array)
- deactivate_task(p, task_rq(p));
- __setscheduler(p, SCHED_NORMAL, 0);
- if (array) {
- __activate_task(p, task_rq(p));
- resched_task(rq->curr);
+ array = p->array;
+ if (array)
+ deactivate_task(p, task_rq(p));
+ __setscheduler(p, SCHED_NORMAL, 0);
+ if (array) {
+ __activate_task(p, task_rq(p));
+ resched_task(rq->curr);
+ }
+
+ task_rq_unlock(rq, &flags);
+ } else if (TASK_NICE(p) == -20) {
+ set_user_nice(p, 0);
}
-
- task_rq_unlock(rq, &flags);
}
read_unlock_irq(&tasklist_lock);
}

--
Måns Rullgård
mru@inprovide.com
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