blk-mq: use sbq wait queues instead of restart for driver tags
Commit 50e1dab8 ("blk-mq-sched: fix starvation for multiple hardware queues and shared tags") fixed one starvation issue for shared tags. However, we can still get into a situation where we fail to allocate a tag because all tags are allocated but we don't have any pending requests on any hardware queue. One solution for this would be to restart all queues that share a tag map, but that really sucks. Ideally, we could just block and wait for a tag, but that isn't always possible from blk_mq_dispatch_rq_list(). However, we can still use the struct sbitmap_queue wait queues with a custom callback instead of blocking. This has a few benefits: 1. It avoids iterating over all hardware queues when completing an I/O, which the current restart code has to do. 2. It benefits from the existing rolling wakeup code. 3. It avoids punting to another thread just to have it block. Signed-off-by: Omar Sandoval <osandov@fb.com> Signed-off-by: Jens Axboe <axboe@fb.com>
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