linux-kernel - Re: [PATCH 0/3] blk-mq & nvme: introduce .map

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Date:	Tue, 29 Sep 2015 08:47:06 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Keith Busch <keith.busch@...el.com>,
	Ming Lei <tom.leiming@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Matthew Wilcox <willy@...ux.intel.com>,
	linux-nvme@...ts.infradead.org, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 0/3] blk-mq & nvme: introduce .map_changed

On 09/29/2015 08:26 AM, Keith Busch wrote:
> On Mon, 28 Sep 2015, Ming Lei wrote:
>> This patchset introduces .map_changed callback into 'struct blk_mq_ops',
>> and use this callback to get NVMe notified about the mapping changed
>> event,
>> then NVMe can update the irq affinity hint for its queues.
>
> I think this is going the wrong direction. Shouldn't we provide blk-mq
> the vectors in the tag set so that layer can manage the irq hints?
>
> This could lead to more cpu-queue assignment optimizations from using
> that information. For example, two h/w contexts sharing the same vector
> shouldn't be assigned to cpus on different NUMA nodes.

I agree, this is moving in the wrong direction. Currently the sw <->hw 
queue mappings are in blk-mq, and this is the exact same information 
base we need for IRQ affinity handling. We need to move in the direction 
of having blk-mq helpers handle that part too, not pass notifications to 
the lower level driver to update its IRQ mappings.

>> Also the 'cpumask' in 'struct blk_mq_tags' isn't needed any more, so
>> remove
>> that and related kernel interface.
>
> It was added to the tags because the cpu mask is an artifact of the
> tags rather that duplicating it across all the h/w contexts sharing the
> same set. It also doesn't let a h/w context from one namespace overwrite
> another's cpu affinity mask when they share the same vector.

So having the mask in the tags is really odd, it should be in some 
per-device type data instead.

-- 
Jens Axboe

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