linux-kernel - Re: [PATCH] irq: revert non-working patch to affinity defaults

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Date:	Fri, 3 Apr 2015 17:13:36 -0700
From:	Jesse Brandeburg <jesse.brandeburg@...el.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	<torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	<linux-kernel@...r.kernel.org>, John <jw@...learfallout.net>
Subject: Re: [PATCH] irq: revert non-working patch to affinity defaults

On Fri, 3 Apr 2015 08:55:57 +0200
Ingo Molnar <mingo@...nel.org> wrote:
> So the original commit also has the problem that it unnecessary 
> drops/retakes the descriptor lock:
> 
> >  	irq_put_desc_unlock(desc, flags);
> > -	/* set the initial affinity to prevent every interrupt being on CPU0 */
> > -	if (m)
> > -		__irq_set_affinity(irq, m, false);
> 
> 
> i.e. why not just call into irq_set_affinity_locked() while we still 
> have the descriptor locked?

I had tried that but it didn't help much.  I also tried kzalloc a new
descriptor like the proc functionality does, and that changes the
behavior a little, but doesn't fix it AFAICS.
 
> Now this is just a small annoyance that should not really matter - it 
> would be nice to figure out the real reason for why the irqs move back 
> to CPU#0.
> 
> In theory the same could happen to 'irqbalanced' as well, if it calls 
> shortly after an irq was registered - so this is not a bug we want to 
> ignore.

Let me know if I can do something to help, the IRQ code is a bit of a
steep learning curve, so the chances of me fixing it are small.
 
> Also, worst case we are back to where v3.19 was, right? So could we 
> try to analyze this a bit more?

Yes, 3.19 shipped with this issue.  Again, let me know if I can help.

Thanks,
 Jesse
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