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

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Date:	Tue, 8 May 2007 09:40:33 +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 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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