Re: [PATCH v2] block: Do not pull requests from the scheduler when we cannot dispatch them

[Hannes Reinecke <hare@suse.de>]

On 6/3/21 12:47 PM, 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.
> 
> Reviewed-by: Ming Lei <ming.lei@redhat.com>
> 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(-)
> 
> Jens, can you please merge the patch? Thanks!
> 
> 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)

Doesn't this lead to a double accounting of the allocated tags?
From what I can see we don't really check if the tag is already
allocated in blk_mq_get_driver_tag() ...

Hmm?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		        Kernel Storage Architect
hare@suse.de			               +49 911 74053 688
SUSE Software Solutions Germany GmbH, 90409 Nürnberg
GF: F. Imendörffer, HRB 36809 (AG Nürnberg)