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@xxxxxxxxx>
>
> 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
