Re: [GIT PATCH] block: unify request processing model and implement peek/fetch

Jens Axboe
2009-05-11T07:53:14Z

On Fri, May 08 2009, Tejun Heo wrote:
> Hello,
>
> Upon ack, please pull from the following git tree.
>
> git://git.kernel.org/pub/scm/linux/kernel/git/tj/misc.git block-peek-fetch
>
> Block layer has allowed two different models of request processing.
> elv_next_request() is used to peek at the top of the queue, after
> peeking, a LLD could start processing it immediately or dequeue and
> then start processing.
>
> The non-dequeuing behavior is mostly useful for simpler device drivers
> (usually PIO based) which process requests on segment basis. By using
> the block layer queue tip as the current request pointer, they don't
> have to care about request boundaries and just process things
> segment-by-segment.
>
> However, this dual mode of operations complicates and ambiguates block
> layer API. Block layer can't tell whether a request has begun
> processing or not in deterministic manner. This makes accounting
> inaccurate and implementing high level features in block layer
> difficult. For example, it isn't clear when a block layer timeout
> timer should be started or how queue queiscing for EH should be
> implemented. Even when problems can be worked aroudn, it makes the
> implementation fragile.
>
> Although allowing llds ignore request boundaries makes things simpler
> for certain drivers, the number of drivers benefit form it aren't too
> many and driver stacks which are even mildly complex have to deal with
> request boundaries anyway. Also, the benefit itself isn't that
> significant. In most cases, it is just another way of doing things
> rather than the definitively better way. IOW, if there were no such
> alternative, nobody would have missed it.
>
> This patchset converts all block layer llds to dequeuing model and
> then clean up API to simplify a bit and enforce dequeueing model.
> This patchset contains the following patches.

Glad this finally got completed, thanks a lot Tejun! Applied.

--
Jens Axboe