Date: Tue, 8 May 2007 09:40:33 +0530
From: Srivatsa Vaddagiri <vatsa@in.ibm.com>
To: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>,
       Linux Kernel <linux-kernel@vger.kernel.org>,
       Zwane Mwaikambo <zwane@arm.linux.org.uk>,
       Nathan Lynch <nathanl@austin.ibm.com>,
       Joel Schopp <jschopp@austin.ibm.com>, Ashok Raj <ashok.raj@intel.com>,
       Heiko Carstens <heiko.carstens@de.ibm.com>,
       Gautham R Shenoy <ego@in.ibm.com>, Ingo Molnar <mingo@elte.hu>,
       paulmck@us.ibm.com
Subject: Re: [BUG] cpu-hotplug: Can't offline the CPU with naughty	realtime	processes
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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).

The other easier option is to add a wake_up_process_to_front() API in
sched.c, which essentially wakes up the process and enqueues the task to
the front of runqueue.

> # Hence this problem is complicated ;-(

-- 
Regards,
vatsa
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