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SubjectRe: [PATCHSET v4] blk-mq-scheduling framework

> Il giorno 17 dic 2016, alle ore 01:12, Jens Axboe <> ha scritto:
> This is version 4 of this patchset, version 3 was posted here:
> From the discussion last time, I looked into the feasibility of having
> two sets of tags for the same request pool, to avoid having to copy
> some of the request fields at dispatch and completion time. To do that,
> we'd have to replace the driver tag map(s) with our own, and augment
> that with tag map(s) on the side representing the device queue depth.
> Queuing IO with the scheduler would allocate from the new map, and
> dispatching would acquire the "real" tag. We would need to change
> drivers to do this, or add an extra indirection table to map a real
> tag to the scheduler tag. We would also need a 1:1 mapping between
> scheduler and hardware tag pools, or additional info to track it.
> Unless someone can convince me otherwise, I think the current approach
> is cleaner.
> I wasn't going to post v4 so soon, but I discovered a bug that led
> to drastically decreased merging. Especially on rotating storage,
> this release should be fast, and on par with the merging that we
> get through the legacy schedulers.

I'm to modifying bfq. You mentioned other missing pieces to come. Do
you already have an idea of what they are, so that I am somehow
prepared to what won't work even if my changes are right?


> Changes since v3:
> - Keep the blk_mq_free_request/__blk_mq_free_request() as the
> interface, and have those functions call the scheduler API
> instead.
> - Add insertion merging from unplugging.
> - Ensure that RQF_STARTED is cleared when we get a new shadow
> request, or merging will fail if it is already set.
> - Improve the blk_mq_sched_init_hctx_data() implementation. From Omar.
> - Make the shadow alloc/free interface more usable by schedulers
> that use the software queues. From Omar.
> - Fix a bug in the io context code.
> - Put the is_shadow() helper in generic code, instead of in mq-deadline.
> - Add prep patch that unexports blk_mq_free_hctx_request(), it's not
> used by anyone.
> - Remove the magic '256' queue depth from mq-deadline, replace with a
> module parameter, 'queue_depth', that defaults to 256.
> - Various cleanups.

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