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

[Satoru Takeuchi]

At Wed, 09 May 2007 10:47:50 +1000,
Nick Piggin wrote:
> 
> 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.

For the time being, I'll try to write the patch implement this idea after
submitting stop_machine_run() fix code. Probably I'll post RFC in one week.

Thanks,
Satoru

> 
> -- 
> SUSE Labs, Novell Inc.
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