Re: [PATCH 1/2] blk-mq: use sbq wait queues instead of restart for driver tags

[Jens Axboe <axboe@fb.com>]

On 02/17/2017 06:05 PM, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
> 
> Commit 50e1dab86aa2 ("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.

This is a great and innovative solution to this problem, it's much
better than stacked restart bits. Thanks Omar, I'll queue this up
for testing.

-- 
Jens Axboe