linux-kernel - Re: [PATCH 06/13] irq: add a helper spread an affinity mask for MSI/MSI-X vectors

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Date:	Wed, 15 Jun 2016 12:10:45 +0200
From:	Christoph Hellwig <hch@....de>
To:	"Guilherme G. Piccoli" <gpiccoli@...ux.vnet.ibm.com>
Cc:	Christoph Hellwig <hch@....de>, tglx@...utronix.de, axboe@...com,
	linux-block@...r.kernel.org, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org
Subject: Re: [PATCH 06/13] irq: add a helper spread an affinity mask for
	MSI/MSI-X vectors

On Tue, Jun 14, 2016 at 06:54:22PM -0300, Guilherme G. Piccoli wrote:
> On 06/14/2016 04:58 PM, Christoph Hellwig wrote:
>> This is lifted from the blk-mq code and adopted to use the affinity mask
>> concept just intruced in the irq handling code.
>
> Very nice patch Christoph, thanks. There's a little typo above, on 
> "intruced".

fixed.

> Another little typo above in "assining".

fixed a swell.

> I take this opportunity to ask you something, since I'm working in a 
> related code in a specific driver

Which driver?  One of the points here is to get this sort of code out
of drivers and into common code..

> - sorry in advance if my question is 
> silly or if I misunderstood your code.
>
> The function irq_create_affinity_mask() below deals with the case in which 
> we have nr_vecs < num_online_cpus(); in this case, wouldn't be a good idea 
> to trying distribute the vecs among cores?
>
> Example: if we have 128 online cpus, 8 per core (meaning 16 cores) and 64 
> vecs, I guess would be ideal to distribute 4 vecs _per core_, leaving 4 
> CPUs in each core without vecs.

There have been some reports about the blk-mq IRQ distribution being
suboptimal, but no one sent patches so far.  This patch just moves the
existing algorithm into the core code to be better bisectable.

I think an algorithm that takes cores into account instead of just SMT
sibling would be very useful.  So if you have a case where this helps
for you an incremental patch (or even one against the current blk-mq
code for now) would be appreciated.