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.
> 
> 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;
> +	} while (count < max_dispatch);
>  
>  	if (!count) {
>  		if (run_queue)
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index c86c01bfecdb..bc2cf80d2c3b 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1100,7 +1100,7 @@ static bool __blk_mq_get_driver_tag(struct request *rq)
>  	return true;
>  }
>  
> -static bool blk_mq_get_driver_tag(struct request *rq)
> +bool blk_mq_get_driver_tag(struct request *rq)
>  {
>  	struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
>  
> diff --git a/block/blk-mq.h b/block/blk-mq.h
> index 9ce64bc4a6c8..81a775171be7 100644
> --- a/block/blk-mq.h
> +++ b/block/blk-mq.h
> @@ -259,6 +259,8 @@ static inline void blk_mq_put_driver_tag(struct request *rq)
>  	__blk_mq_put_driver_tag(rq->mq_hctx, rq);
>  }
>  
> +bool blk_mq_get_driver_tag(struct request *rq);
> +
>  static inline void blk_mq_clear_mq_map(struct blk_mq_queue_map *qmap)
>  {
>  	int cpu;

Thinking of further, looks this patch is fine, and it is safe to use driver tag
allocation result to decide if more requests need to be dequeued since run queue
always be followed when breaking from the loop. Also I can observe that
io.bfq.weight is improved on virtio-blk, so

Reviewed-by: Ming Lei <ming.lei@redhat.com>

Thanks,
Ming