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 10:35:21 +0200
From:	Bart Van Assche <bart.vanassche@...disk.com>
To:	"Guilherme G. Piccoli" <gpiccoli@...ux.vnet.ibm.com>,
	Christoph Hellwig <hch@....de>
CC:	<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 06/14/2016 11:54 PM, Guilherme G. Piccoli wrote:
> On 06/14/2016 04:58 PM, Christoph Hellwig wrote:
> I take this opportunity to ask you something, since I'm working in a
> related code in a specific driver - 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.

Hello Christoph and Guilherme,

I would also like to see irq_create_affinity_mask() modified such that 
it implements Guilherme's algorithm. I think blk-mq requests should be 
processed by a CPU core from the NUMA node from which the request has 
been submitted. With the proposed algorithm, if the number of MSI-X 
vectors is less than or equal to the number of CPU cores of a single 
node, all interrupt vectors will be assigned to the first NUMA node.

Bart.