Re: [PATCH v2 03/12] block: Send requeued requests to the I/O scheduler

[Christoph Hellwig <hch@lst.de>]

On Fri, Apr 07, 2023 at 04:58:13PM -0700, Bart Van Assche wrote:
> +		blk_mq_sched_insert_request(rq, /*at_head=*/true, false, false);

The whole usage of at_head in the request_list-related code looks
suspicious to me.

All callers of blk_mq_add_to_requeue_list except for blk_kick_flush
pass at_head=true.  So the request_list is basically LIFO except for
that one caller.

blk_mq_requeue_work than does a HEAD insert for them, unless they
are marked RQF_DONTPREP because the driver already did some setup.
So except for the RQF_DONTPREP we basically revert the at_head insert.

This all feels wrong to me.  I think we need to get to a point where
the request_list itself is always added to at the tail, processed
head to tail, but inserted into the scheduler or the hctx rq_list
before other pending requests, probaly using similar code as
blk_mq_flush_plug_list / blk_mq_dispatch_plug_list.