linux-kernel - Re: [CPUISOL] CPU isolation extensions

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Date:	Mon, 28 Jan 2008 11:34:50 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Paul Jackson <pj@....com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>, maxk@...lcomm.com,
	linux-kernel@...r.kernel.org, mingo@...e.hu, srostedt@...hat.com,
	ghaskins@...ell.com
Subject: Re: [CPUISOL] CPU isolation extensions

On Mon, Jan 28, 2008 at 08:59:10AM -0600, Paul Jackson wrote:
> Thanks for the CC, Peter.

Thanks from me too.

> Max wrote:
> > 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.
> 
> I recently added the per-cpuset flag 'sched_load_balance' for some
> other realtime folks, so that they can disable the kernel scheduler
> load balancing on isolated CPUs.  It essentially allows for dynamic
> control of which CPUs are isolated by the scheduler, using the cpuset
> hierarchy, rather than enhancing the 'isolated_cpus' mask.   That
> 'isolated_cpus' mask remained a minimal kernel boottime parameter.
> I believe this went to Linus's tree about Oct 2007.
> 
> It looks like you have three additional tweaks for realtime in this
> patch set, with your patches:
> 
>   [PATCH] [CPUISOL] Do not route IRQs to the CPUs isolated at boot

I didn't know we still routed IRQs to isolated CPUs. I guess I need to
look deeper into the code on this one. But I agree that isolated CPUs
should not have IRQs routed to them.

>   [PATCH] [CPUISOL] Support for workqueue isolation

The thing about workqueues is that they should only be woken on a CPU if
something on that CPU accessed them. IOW, the workqueue on a CPU handles
work that was called by something on that CPU. Which means that
something that high prio task did triggered a workqueue to do some work.
But this can also be triggered by interrupts, so by keeping interrupts
off the CPU no workqueue should be activated.

>   [PATCH] [CPUISOL] Isolated CPUs should be ignored by the "stop machine"

This I find very dangerous. We are making an assumption that tasks on an
isolated CPU wont be doing things that stopmachine requires. What stops
a task on an isolated CPU from calling something into the kernel that
stop_machine requires to halt?

-- Steve


> 
> It would be interesting to see a patchset with the above three realtime
> tweaks, layered on this new cpuset 'sched_load_balance' apparatus, rather
> than layered on changes to make 'isolated_cpus' more dynamic.  Some of us
> run realtime and cpuset-intensive loads on the same system, so like to
> have those two capabilities co-operate with each other.
> 
> Ingo - what's your sense of the value of the above three realtime tweaks
>        (the last three patches in Max's patch set)?
> 
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