linux-kernel - Re: IRQ affinities

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Message-ID: <48337792.20606@qualcomm.com>
Date:	Tue, 20 May 2008 18:14:58 -0700
From:	Max Krasnyanskiy <maxk@...lcomm.com>
To:	Paul Jackson <pj@....com>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>, menage@...gle.com,
	mingo@...e.hu, linux-kernel@...r.kernel.org
Subject: Re: IRQ affinities

Paul Jackson wrote:
> Peter wrote:
>> That's a new feature; and its quite common that new features require
>> code changes.
> 
> It's common for new features to require code changes to take advantage
> of the new features.
> 
> It's less desirable that taking advantage of such new features breaks
> existing, basically unrelated, code.
> 
> My gut sense is that, in a misguided effort to find a "simple" answer
> to irq distribution, we (well, y'all) are trying to attach this
> feature to cpusets or cgroups.
> 
> Let me ask a different question:
> 
>   What solutions would you (Max, Peter, Ingo, lurkers, ...) be
>   suggesting for this 'IRQ affinity' problem if cpusets and
>   cgroups didn't exist in any form whatsoever?

As Peter explained I'm focusing on the "CPU isolation" aspect. ie Shielding a 
CPU (or a set of CPUs) from various kernel activities (load balancing, soft 
and hard irq handling, workqueues, etc).

For the IRQs specifically all I need is to be able to tell the kernel to not 
route IRQs to certain CPUs. That's mostly works already via 
/proc/irq/N/smp_affinity, the problem is dynamically allocated irqs because 
/proc/irq/N directory does not exist until those IRQs are allocated/enabled.

Originally I introduced global cpu_isolated_map. IRQ code was using that map 
to exclude CPU(s) from IRQ routing. What I realized now is that all I need is
/proc/irq/default_smp_affinity. In other words I just need to export default 
mask used by the IRQ layer. I think this makes sense regardless of what cpuset 
  based solution we'll come up with.

Max
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