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

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Message-ID: <874pmoro1c.wl%takeuchi_satoru@jp.fujitsu.com>
Date:	Tue, 08 May 2007 12:29:19 +0900
From:	Satoru Takeuchi <takeuchi_satoru@...fujitsu.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Satoru Takeuchi <takeuchi_satoru@...fujitsu.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	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>
Subject: Re: [BUG] cpu-hotplug: Can't offline the CPU with naughty	realtime	processes

At Tue, 08 May 2007 13:02:25 +1000,
Rusty Russell wrote:
> 
> On Tue, 2007-05-08 at 11:41 +0900, Satoru Takeuchi wrote:
> > At Mon, 07 May 2007 23:42:53 +1000,
> > Rusty Russell wrote:
> > > I look forward to your patch!
> > > Rusty.
> > 
> > Thanks, I'll do. Maybe this work will take several days including test.
> 
> Excellent.
> 
> > BTW, how should I manage rt process having max priority as Gautham said?
> > He said that it's OK unless such kernel thread exists. However, currently
> > MAX_USER_RT_PRIORITY is equal to MAX_RT_PRIO, so user process also be able
> > to cause this problem. Is Srivatsa's idea 2 acceptable? Or just apply
> > "Shouldn't abuse highest rt proority" rule?
> 
> 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.

# Hence this problem is complicated ;-(

Thanks,

Satoru
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