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Date: Wed, 3 Feb 2016 20:15:15 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Rik van Riel <riel@redhat.com>
cc: LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Chris Metcalf <cmetcalf@ezchip.com>, Christoph Lameter <cl@linux.com>
Subject: Re: [PATCH] genirq: Add default affinity mask command line option
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On Wed, 3 Feb 2016, Rik van Riel wrote:
> On 02/03/2016 01:52 PM, Thomas Gleixner wrote:
> > From: Thomas Gleixner <tglx@linutronix.de> Date: Fri May 25
> > 16:59:47 2012 +0200 Subject: genirq: Add default affinity mask
> > command line option
> > 
> > If we isolate CPUs, then we don't want random device interrupts on
> > them. Even w/o the user space irq balancer enabled we can end up
> > with irqs on non boot cpus and chasing newly requested interrupts
> > is a tedious task.
> 
> The userspace irqbalance daemon has code nowadays to prevent
> it from putting irqs on isolated CPUs.

I know, but a lot of systems do not enable it at all.

> > Allow to restrict the default irq affinity mask.
> 
> However, not placing them on there at boot time is a great
> enhancement, too.

That's the main plan :)
 
> Would it make sense to key off the isolated_cpus mask
> if isolated_cpus= was specified, but no irqaffinity
> mask was specified?

That might be an add on.
 
> Is there any use case where we would want irqs on
> isolated cpus by default, and could not bear the
> cost of reassigning them from userspace after boot?

I don't think so. Those interrupts you want on an isolated cpu are obviously
hand selected.

Thanks,

	tglx