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

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Message-ID: <87irb3g4zt.wl%takeuchi_satoru@jp.fujitsu.com>
Date:	Tue, 08 May 2007 16:16:06 +0900
From:	Satoru Takeuchi <takeuchi_satoru@...fujitsu.com>
To:	vatsa@...ibm.com
Cc:	Satoru Takeuchi <takeuchi_satoru@...fujitsu.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Zwane Mwaikambo <zwane@....linux.org.uk>,
	Nathan Lynch <nathanl@...tin.ibm.com>,
	Joel Schopp <jschopp@...tin.ibm.com>,
	Ashok Raj <ashok.raj@...el.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Gautham R Shenoy <ego@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>, paulmck@...ibm.com
Subject: Re: [BUG] cpu-hotplug: Can't offline the CPU with naughty	realtime	processes

At Tue, 8 May 2007 09:40:33 +0530,
Srivatsa Vaddagiri wrote:
> 
> On Tue, May 08, 2007 at 12:29:19PM +0900, Satoru Takeuchi wrote:
> > > We used to be able to create kernel threads higher than any userspace
> > > priority.  If this is no longer true, I think that's OK: equal priority
> > > still means we'll get scheduled, right?
> > 
> > IF SCHED_RR, yes. However, if SCHED_FIFO, no. Such process doen't have timeslice
> > and only relinquish CPU time voluntarily.
> 
> yeah ..this is truly a problem if SCHED_FIFO user-space cpu hog task is
> running at MAX_USER_RT_PRIO (which happens to be same as max real-time
> priority kernel threads can attain - MAX_RT_PRIO).
> 
> One option is to make MAX_USER_RT_PRIO < MAX_RT_PRIO. I am not sure what
> semantics that will break (perhaps the real-time folks can clarify
> that).

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.           |
+-----------+-----------------------------------------------+
| 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,

Satoru
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