linux-kernel - Re: [RFC] blk-mq: support for shared tags

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Date:	Thu, 3 Apr 2014 20:01:43 +0200
From:	Christoph Hellwig <hch@....de>
To:	Matias Bjorling <m@...rling.me>
Cc:	Jens Axboe <axboe@...com>, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org
Subject: Re: [RFC] blk-mq: support for shared tags

On Thu, Apr 03, 2014 at 09:45:11AM -0700, Matias Bjorling wrote:
> > I'd still create a request_queue for the internal queue, just not register
> > a block device for it.  For example SCSI sets up queues for each LUN
> > found, but only a subset actually is exposed as a block device.
> > 
> 
> Ok. That is good enough for now. A little heavy on the overhead side, if
> only the tag logic is needed.
> 
> What about the following suggestions for shared tags:
> 
> 1. Rename it from blk_mq_shared_tags to blk_mq_tag_group. A driver can
> have several tag groups that it maintains.

I was going to rename it to tag_set, but tag_group sounds fine to me as well.

> 2. Instead of blk_mq_shared_tags structure in blk_mq_reg. Have  function
> pointer for getting the tags structure during hctx initialization. This
> is interesting for nvme, because it has as set of tags for each hardware
> queue it exposes.

The current code also has an array of blk_mq_tags structures, one for
each queue.  Do you need a more complicated mapping than that?

Btw, I was also going to siply split out the tag allocation from the
queue registration unconditionally.  While this adds a little more
boilerplate to simple drivers it avoids unconditional code pathes and should
make the model much easier to understand.  I should have a new version
of the patches soon.

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