Re: [BUG] cpu-hotplug: Can't offline the CPU with naughty realtimeprocesses

[Nick Piggin]

Satoru Takeuchi wrote:
> 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 :-)

We've seen the same problem with other stop_machine_run sites in the kernel.
module remove was one.

Reserving the top priority slot for stop machine (and migration thread, I
guess) isn't a bad idea.

--
SUSE Labs, Novell Inc.
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