Date: Tue, 29 Mar 2016 10:44:37 -0700
From: Christoph Hellwig <hch@infradead.org>
To: Jens Axboe <axboe@fb.com>
Cc: Shaohua Li <shli@fb.com>, Christoph Hellwig <hch@infradead.org>,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        Kernel-team@fb.com
Subject: Re: [PATCH 1/3] blk-mq: add an API to estimate hardware queue node
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On Tue, Mar 29, 2016 at 10:50:11AM -0600, Jens Axboe wrote:
> >This looks weird, shouldn't the cpu assignment be determined by block
> >core (blk-mq) because block core decides how to use the queue?
> 
> I agree, that belongs in the blk-mq proper, the driver should just follow
> the rules outlined, not impose their own in this regard. It'll also help
> with irq affinity mappings, once we get that in.

It's not going to work that way unfortunately.  Lots of driver simply
have no control over the underlying interrupts.  Think of any RDMA
storage or other layer drivers - they get low level queues from a layer
they don't control and need a block queue for each of them.

My plan is to make the block layer follow what the networking layer
does - get the low level queues / MSI-X pairs and then use the
infrastructure in lib/cpu_rmap.c to figure out the number of queues
and queue placement for them.

The lower half of that is about to get rewritten by Thomas after
disccussion between him, me and a few others to provide drivers
nice APIs for spreading MSI-X vectors over CPUs or nodes.