Message-ID: <47A1BC0D.3040404@cfl.rr.com>
Date:	Thu, 31 Jan 2008 07:16:13 -0500
From:	Mark Hounschell <dmarkh@....rr.com>
To:	maxk@...lcomm.com
CC:	linux-kernel@...r.kernel.org, Mark Hounschell <dmarkh@....rr.com>
Subject: Re: [CPUISOL] CPU isolation extensions

maxk@...lcomm.com wrote:
> Following patch series extends CPU isolation support. Yes, most people want to virtuallize 
> CPUs these days and I want to isolate them :).
> The primary idea here is to be able to use some CPU cores as dedicated engines for running
> user-space code with minimal kernel overhead/intervention, think of it as an SPE in the 
> Cell processor.
> 
> We've had scheduler support for CPU isolation ever since O(1) scheduler went it. 
> I'd like to extend it further to avoid kernel activity on those CPUs as much as possible.
> In fact that the primary distinction that I'm making between say "CPU sets" and 
> "CPU isolation". "CPU sets" let you manage user-space load while "CPU isolation" provides
> a way to isolate a CPU as much as possible (including kernel activities).
> 
> I'm personally using this for hard realtime purposes. With CPU isolation it's very easy to 
> achieve single digit usec worst case and around 200 nsec average response times on off-the-shelf
> multi- processor/core systems under exteme system load. I'm working with legal folks on releasing 
> hard RT user-space framework for that.
> I can also see other application like simulators and stuff that can benefit from this.
> 
> I've been maintaining this stuff since around 2.6.18 and it's been running in production
> environment for a couple of years now. It's been tested on all kinds of machines, from NUMA
> boxes like HP xw9300/9400 to tiny uTCA boards like Mercury AXA110.
> The messiest part used to be SLAB garbage collector changes. With the new SLUB all that mess 
> goes away (ie no changes necessary). Also CFS seems to handle CPU hotplug much better than O(1) 
> did (ie domains are recomputed dynamically) so that isolation can be done at any time (via sysfs). 
> So this seems like a good time to merge. 
> 
> Anyway. The patchset consist of 5 patches. First three are very simple and non-controversial.
> They simply make "CPU isolation" a configurable feature, export cpu_isolated_map and provide
> some helper functions to access it (just like cpu_online() and friends).
> Last two patches add support for isolating CPUs from running workqueus and stop machine.
> More details in the individual patch descriptions.
> 
> Ideally I'd like all of this to go in during this merge window. If people think it's acceptable 
> Linus or Andrew (or whoever is more appropriate Ingo maybe) can pull this patch set from
> 	git://git.kernel.org/pub/scm/linux/kernel/git/maxk/cpuisol-2.6.git
> 

It's good to see hear from someone else that thinks a multi-processor
box _should_ be able to run a CPU intensive (%100) RT app on one of the
processors without adversely affecting or being affected by the others.
I have had issues that were _traced_ back to the fact that I am doing
just that. All I got was, you can't do that or we don't support that
kind of thing in the Linux kernel.

One example,  Andrew Mortons feedback to the LKML thread "floppy.c soft
lockup"

Good luck with this. I hope this gets someones attention.

BTW, I have tried your patches against a vanilla 2.6.24 kernel but am
not successful.

# echo '1' > /sys/devices/system/cpu/cpu1/isolated
bash: echo: write error: Device or resource busy

The cpuisol=1 cmdline option yields:

harley:# cat /sys/devices/system/cpu/cpu1/isolated
0

harley:# cat /proc/cmdline
root=/dev/sda3 vga=normal apm=off selinux=0 noresume splash=silent
kmalloc=192M cpuisol=1




Regards
Mark



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