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

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Date:	Tue, 29 Sep 2015 14:26:07 +0000 (UTC)
From:	Keith Busch <keith.busch@...el.com>
To:	Ming Lei <tom.leiming@...il.com>
cc:	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
	Keith Busch <keith.busch@...el.com>,
	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 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.

> 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.
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