Re: [PATCH v2 3/3] block: Improve performance for BLK_MQ_F_BLOCKING drivers

[Christoph Hellwig <hch@lst.de>]

On Wed, Jul 19, 2023 at 11:22:42AM -0700, Bart Van Assche wrote:
> blk_mq_run_queue() runs the queue asynchronously if BLK_MQ_F_BLOCKING
> has been set.

Maybe something like:

blk_mq_run_queue() always runs the queue asynchronously if
BLK_MQ_F_BLOCKING is set on the tag_set.

> + *    for execution. Don't wait for completion. May sleep if BLK_MQ_F_BLOCKING
> + *    has been set.
>   *
>   * Note:
>   *    This function will invoke @done directly if the queue is dead.
> @@ -2213,6 +2214,8 @@ void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
>  	 */
>  	WARN_ON_ONCE(!async && in_interrupt());
>  
> +	might_sleep_if(!async && hctx->flags & BLK_MQ_F_BLOCKING);

This is some odd an very complex calling conventions.  I suspect most
!BLK_MQ_F_BLOCKING could also deal with the may sleep if not async,
and that would give us a much easier to audit change as we could
remove the WARN_ON_ONCE above and just do a:

	might_sleep_if(!async);

In fact this might be a good time to split up blk_mq_run_hw_queue
into blk_mq_run_hw_queue and blk_mq_run_hw_queue_async and do
away with the bool and have cristal clear calling conventions.

If we really need !async calles than can sleep we can add a specific
blk_mq_run_hw_queue_atomic.