Re: [PATCH] blk-mq: don't queue more if we get a busy return

[Ming Lei <ming.lei@redhat.com>]

On Thu, Jun 28, 2018 at 09:46:50AM -0600, Jens Axboe wrote:
> Some devices have different queue limits depending on the type of IO. A
> classic case is SATA NCQ, where some commands can queue, but others
> cannot. If we have NCQ commands inflight and encounter a non-queueable
> command, the driver returns busy. Currently we attempt to dispatch more
> from the scheduler, if we were able to queue some commands. But for the
> case where we ended up stopping due to BUSY, we should not attempt to
> retrieve more from the scheduler. If we do, we can get into a situation
> where we attempt to queue a non-queueable command, get BUSY, then
> successfully retrieve more commands from that scheduler and queue those.
> This can repeat forever, starving the non-queuable command indefinitely.
> 
> Fix this by NOT attempting to pull more commands from the scheduler, if
> we get a BUSY return. This should also be more optimal in terms of
> letting requests stay in the scheduler for as long as possible, if we
> get a BUSY due to the regular out-of-tags condition.
> 
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index b6888ff556cf..d394cdd8d8c6 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1075,6 +1075,9 @@ static bool blk_mq_mark_tag_wait(struct blk_mq_hw_ctx **hctx,
>  
>  #define BLK_MQ_RESOURCE_DELAY	3		/* ms units */
>  
> +/*
> + * Returns true if we did some work AND can potentially do more.
> + */
>  bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
>  			     bool got_budget)
>  {
> @@ -1205,8 +1208,17 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
>  			blk_mq_run_hw_queue(hctx, true);
>  		else if (needs_restart && (ret == BLK_STS_RESOURCE))
>  			blk_mq_delay_run_hw_queue(hctx, BLK_MQ_RESOURCE_DELAY);
> +
> +		return false;
>  	}
>  
> +	/*
> +	 * If the host/device is unable to accept more work, inform the
> +	 * caller of that.
> +	 */
> +	if (ret == BLK_STS_RESOURCE || ret == BLK_STS_DEV_RESOURCE)
> +		return false;

The above change may not be needed since one invariant is that
!list_empty(list) becomes true if either BLK_STS_RESOURCE or BLK_STS_DEV_RESOURCE
is returned from .queue_rq().

Thanks,
Ming