Re: [PATCH] block: Do not pull requests from the scheduler when we cannot dispatch them

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

On Thu, May 20, 2021 at 01:25:28PM +0200, Jan Kara wrote:
> Provided the device driver does not implement dispatch budget accounting
> (which only SCSI does) the loop in __blk_mq_do_dispatch_sched() pulls
> requests from the IO scheduler as long as it is willing to give out any.
> That defeats scheduling heuristics inside the scheduler by creating
> false impression that the device can take more IO when it in fact
> cannot.

So hctx->dispatch_busy isn't set as true in this case?

> 
> For example with BFQ IO scheduler on top of virtio-blk device setting
> blkio cgroup weight has barely any impact on observed throughput of
> async IO because __blk_mq_do_dispatch_sched() always sucks out all the
> IO queued in BFQ. BFQ first submits IO from higher weight cgroups but
> when that is all dispatched, it will give out IO of lower weight cgroups
> as well. And then we have to wait for all this IO to be dispatched to
> the disk (which means lot of it actually has to complete) before the
> IO scheduler is queried again for dispatching more requests. This
> completely destroys any service differentiation.
> 
> So grab request tag for a request pulled out of the IO scheduler already
> in __blk_mq_do_dispatch_sched() and do not pull any more requests if we
> cannot get it because we are unlikely to be able to dispatch it. That
> way only single request is going to wait in the dispatch list for some
> tag to free.
> 
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  block/blk-mq-sched.c | 12 +++++++++++-
>  block/blk-mq.c       |  2 +-
>  block/blk-mq.h       |  2 ++
>  3 files changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> index 996a4b2f73aa..714e678f516a 100644
> --- a/block/blk-mq-sched.c
> +++ b/block/blk-mq-sched.c
> @@ -168,9 +168,19 @@ static int __blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
>  		 * in blk_mq_dispatch_rq_list().
>  		 */
>  		list_add_tail(&rq->queuelist, &rq_list);
> +		count++;
>  		if (rq->mq_hctx != hctx)
>  			multi_hctxs = true;
> -	} while (++count < max_dispatch);
> +
> +		/*
> +		 * If we cannot get tag for the request, stop dequeueing
> +		 * requests from the IO scheduler. We are unlikely to be able
> +		 * to submit them anyway and it creates false impression for
> +		 * scheduling heuristics that the device can take more IO.
> +		 */
> +		if (!blk_mq_get_driver_tag(rq))
> +			break;

At default BFQ's queue depth is same with virtblk_queue_depth, both are
256, so looks you use non-default setting?

Also in case of running out of driver tag, hctx->dispatch_busy should have
been set as true for avoiding batching dequeuing, does the following
patch make a difference for you?


diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index 045b6878b8c5..c2ce3091ad6e 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -107,6 +107,13 @@ static bool blk_mq_dispatch_hctx_list(struct list_head *rq_list)
 
 #define BLK_MQ_BUDGET_DELAY	3		/* ms units */
 
+static int blk_mq_sched_max_disaptch(struct blk_mq_hw_ctx *hctx)
+{
+	if (!hctx->dispatch_busy)
+		return hctx->queue->nr_requests;
+	return 1;
+}
+
 /*
  * Only SCSI implements .get_budget and .put_budget, and SCSI restarts
  * its queue by itself in its completion handler, so we don't need to
@@ -121,15 +128,9 @@ static int __blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
 	struct elevator_queue *e = q->elevator;
 	bool multi_hctxs = false, run_queue = false;
 	bool dispatched = false, busy = false;
-	unsigned int max_dispatch;
 	LIST_HEAD(rq_list);
 	int count = 0;
 
-	if (hctx->dispatch_busy)
-		max_dispatch = 1;
-	else
-		max_dispatch = hctx->queue->nr_requests;
-
 	do {
 		struct request *rq;
 		int budget_token;
@@ -170,7 +171,7 @@ static int __blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
 		list_add_tail(&rq->queuelist, &rq_list);
 		if (rq->mq_hctx != hctx)
 			multi_hctxs = true;
-	} while (++count < max_dispatch);
+	} while (++count < blk_mq_sched_max_disaptch(hctx));
 
 	if (!count) {
 		if (run_queue)


Thanks,
Ming