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SubjectRe: [BUG] cpu-hotplug: Can't offline the CPU with naughty realtime processes
At Tue, 8 May 2007 22:18:50 +0530,
Srivatsa Vaddagiri wrote:
>
> On Tue, May 08, 2007 at 04:16:06PM +0900, Satoru Takeuchi wrote:
> > Sometimes I wonder at prio_array. It has 140 entries(from 0 to 139),
> > and the meaning of each entry is as follows, I think.
> >
> > +-----------+-----------------------------------------------+
> > | index | usage |
> > +-----------+-----------------------------------------------+
> > | 0 - 98 | RT processes are here. They are in the entry |
> > | | whose index is 99 - sched_priority. |
>
> >From sched.h:
>
> /*
> * Priority of a process goes from 0..MAX_PRIO-1, valid RT
> * priority is 0..MAX_RT_PRIO-1, and SCHED_NORMAL/SCHED_BATCH
> * tasks are in the range MAX_RT_PRIO..MAX_PRIO-1.
>
> so shouldn't the index for RT processes be 0 - 99, given that
> MAX_RT_PRIO = 100?

However `man sched_priority' says...


Processes scheduled with SCHED_OTHER or SCHED_BATCH must
be assigned the static priority 0. Processes scheduled
under SCHED_FIFO or SCHED_RR can have a static priority
in the range 1 to 99. The system calls
sched_get_priority_min() and sched_get_priority_max() can
be used to find out the valid priority range for a
scheduling policy in a portable way on all POSIX.1-2001
conforming systems.


and see the kernel/sched.c ...


int sched_setscheduler(struct task_struct *p, int policy,
struct sched_param *param)
{
...
/*
* Valid priorities for SCHED_FIFO and SCHED_RR are
* 1..MAX_USER_RT_PRIO-1, valid priority for SCHED_NORMAL and
* SCHED_BATCH is 0.
*/
if (param->sched_priority < 0 ||
(p->mm && param->sched_priority > MAX_USER_RT_PRIO-1) ||
(!p->mm && param->sched_priority > MAX_RT_PRIO-1))
return -EINVAL;
if (is_rt_policy(policy) != (param->sched_priority != 0))
return -EINVAL;
...
}

So, if I want to set the rt_prio of a kernel_thread, we can't use this
entry unless set t->prio to 99 directly. I don't know whether we are
allowed to write such code bipassing sched_setscheduler(). In addition,
even if kernel_thread can use this index , I can't understand it's usage.
It can only be used by kernel, but its priority is LOWER than any real
time thread.

If the rule can be changed to the following...

+-----------+-----------------------------------------------+
| index | usage |
+-----------+-----------------------------------------------+
| 0 | RT processes are here. Only kernel can use |
| | this entry. |
+-----------+-----------------------------------------------+
| 1 - 99 | RT processes are here. They are in the entry |
| | whose index is 99 - sched_priority. |
+-----------+-----------------------------------------------+
| 100 - 139 | Ordinally processes are here. They are in the |
| | entry whose index is (nice+120) +/- 5 |
+-----------+-----------------------------------------------+
... there will be an entry only used by kernel and its priority is HIGHER
than any user process, and I'll get happy :-)

Thanks,

Satoru

>
> > +-----------+-----------------------------------------------+
> > | 99 | No one use it? CMIIW. |
> > +-----------+-----------------------------------------------+
> > | 100 - 139 | Ordinally processes are here. They are in the |
> > | | entry whose index is (nice+120) +/- 5 |
> > +-----------+-----------------------------------------------+
> >
> > What's the purpose of the prio_array[99]? Once I exlore source tree
> > briefly and can't found any kernel thread which uses this entry.
> > Does anybody know?
>
> --
> Regards,
> vatsa
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