linux-kernel - Re: Inquiry: Should we remove "isolcpus= kernel boot option? (may have realtime uses)

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Date:	Mon, 02 Jun 2008 20:32:17 -0700
From:	Max Krasnyansky <maxk@...lcomm.com>
To:	Ingo Oeser <ioe-lkml@...eria.de>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>, Paul Jackson <pj@....com>,
	linux-kernel@...r.kernel.org, Con Kolivas <kernel@...ivas.org>,
	"Derek L. Fults" <dfults@....com>, devik <devik@....cz>,
	Dimitri Sivanich <sivanich@....com>,
	Dinakar Guniguntala <dino@...ibm.com>,
	Emmanuel Pacaud <emmanuel.pacaud@...v-poitiers.fr>,
	Frederik Deweerdt <deweerdt@...e.fr>,
	Ingo Molnar <mingo@...e.hu>,
	Matthew Dobson <colpatch@...ibm.com>,
	Nick Piggin <nickpiggin@...oo.com.au>, rostedt@...dmis.org,
	Oleg Nesterov <oleg@...sign.ru>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Paul Menage <menage@...gle.com>,
	"Randy.Dunlap" <rddunlap@...l.org>, suresh.b.siddha@...el.com,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: Inquiry: Should we remove "isolcpus= kernel boot option? (may
 have realtime uses)



Ingo Oeser wrote:
> Hi Max,
> Hi Peter,
> 
> On Tuesday 03 June 2008, Max Krasnyansky wrote:
>> Ingo, I just wanted to elaborate on what Peter is saying. That CPU will still
>> have to be _booted_ properly. It may be used for hard- and soft- interrupt
>> processing, workqueues (internal kernel queuing mechanism) and kernel timers.
> 
> Oh! Didn't know that user process scheduling is so much 
Not sure what you meant here. Stuff that I listed has nothing to do with user
process scheduling.

>> In your particular case you're much much much better off with doing
>> 	echo 0 > /sys/devices/system/cpuN/online
>> either during initrd stage or as a first init script.
>> That way bad cpu will be _completely_ disabled.
> 
> The initrd is from the distribution. I have no sane way to change it
> fast and permanent. Can I change the initrd and still have a certified
> RHEL or SLES? Are there initrd hooks, which survive packet installation?
That's why I mentioned "first init" script. You can create a simple init.d
compliant script that runs with priority 0 (see /etc/init.d/network for
example). That should be early enough.

> I would really appreciate some way to keep the kernel from using 
> a CPU at all to do fault isolation. If possible not even booting it.
How does isolcpu= boot option helps in this case ?
I suppose the closes option is maxcpus=. We can probably add ignorecpus= or
something to handle your use case but it has nothing to do with isolcpus=.

> Bootparameters survived all distro fiddling so far. I love them!
So do custom init.d scripts.

> 
> Try to convince a hardware vendor, that you don't have a software bug.
> Try to convince him that you didn't break the hardware by swapping it around.
> 
> So I'll ACK removing isolcpus, if we get a better replacement boot option.
I think you're missing the point here. It's like saying
"Lets not switch to electric cars because I use gasoline to kill weeds".

As I mentioned before, cpus listed in the isolcpus= boot option will still
handle hard-/soft- irqs, kernel work, kernel timers. You are much better off
using cpu hotplug (ie putting bad cpu offline). Feel free to propose
ignorecpus= option in a separate thread.

Max





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