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

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Message-ID: <20070507123956.GA11416@in.ibm.com>
Date:	Mon, 7 May 2007 18:09:56 +0530
From:	Gautham R Shenoy <ego@...ibm.com>
To:	Srivatsa Vaddagiri <vatsa@...ibm.com>
Cc:	Satoru Takeuchi <takeuchi_satoru@...fujitsu.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	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>
Subject: Re: [BUG] cpu-hotplug: Can't offline the CPU with naughty realtime processes

On Mon, May 07, 2007 at 04:32:56PM +0530, Srivatsa Vaddagiri wrote:
> On Mon, May 07, 2007 at 04:17:24PM +0530, Gautham R Shenoy wrote:
> > Nevertheless, with the freezer based approach that we're experimenting,
> > this problem shouldn't arise. We expect the whole system to get frozen
> > before we actually do a cpu_down() (which will then call
> > __stop_machine_run). So any such rogue RT task will have to first fail
> > the freezer ( which it will), 
> 
> >From what I understand of the freezer, if the RT task is running in user
> space (which seems to be the case in this thread), it should get frozen even 
> if it is a forever running SCHED_FIFO task at MAX_RT_PRIO -1 priority?

Yes, you are right. It will end up getting the fake signal. 
So yeah, freezer pretty much solves the problem for cpu hotplug.

But I now wonder if we will have some problem with module stopping if we 
have a high prio SCHED_FIFO in the system.

> 
> -- 
> Regards,
> vatsa

Regards
gautham.
-- 
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"
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