linux-kernel - Re: [PATCH 02/13] irq: Introduce IRQD_AFFINITY

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Date:	Mon, 20 Jun 2016 15:21:47 +0200
From:	Bart Van Assche <bart.vanassche@...disk.com>
To:	Christoph Hellwig <hch@....de>
CC:	Keith Busch <keith.busch@...el.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"axboe@...com" <axboe@...com>,
	"linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 02/13] irq: Introduce IRQD_AFFINITY_MANAGED flag

On 06/20/2016 02:22 PM, Christoph Hellwig wrote:
> On Thu, Jun 16, 2016 at 05:39:07PM +0200, Bart Van Assche wrote:
>> On 06/16/2016 05:20 PM, Christoph Hellwig wrote:
>>> On Wed, Jun 15, 2016 at 09:36:54PM +0200, Bart Van Assche wrote:
>> My concern
>> is that I doubt that there is an interrupt assignment scheme that works
>> optimally for all workloads. Hence my request to preserve the ability to
>> modify interrupt affinity from user space.
>
> I'd say let's do such an interface incrementally based on the use
> case - especially after we get networking over to use common code
> to distribute the interrupts.  If you were doing something like this
> with the current blk-mq code it wouldn't work very well due to the
> fact that you'd have a mismatch between the assigned interrupt and
> the blk-mq queue mapping anyway.
>
> It might be a good idea to start brainstorming how we'd want to handle
> this change - we'd basically need a per-device notification that the
> interrupt mapping changes so that we can rebuild the queue mapping,
> which is somewhat similar to the lib/cpu_rmap.c code used by a few
> networking drivers.  This would also help with dealing with cpu
> hotplug events that change the cpu mapping.

A notification mechanism that reports interrupt mapping changes will 
definitely help. What would also help is an API that allows drivers to 
query the MSI-X IRQ of an adapter that is nearest given a cpumask, e.g. 
hctx->cpumask. Another function can then map that IRQ into an index in 
the range 0..n-1 where n is the number of MSI-X interrupts for that 
adapter. Every blk-mq/scsi-mq driver will need this functionality to 
decide which IRQ to associate with a block layer hctx.

Bart.