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

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Message-ID: <20070508164850.GS7311@in.ibm.com>
Date:	Tue, 8 May 2007 22:18:50 +0530
From:	Srivatsa Vaddagiri <vatsa@...ibm.com>
To:	Satoru Takeuchi <takeuchi_satoru@...fujitsu.com>
Cc:	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

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?

> +-----------+-----------------------------------------------+
> | 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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