Re: [PATCH 0/3] blk-mq & nvme: introduce .map_changed

From: Jens Axboe
Date: Tue Sep 29 2015 - 10:47:53 EST

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
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
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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