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@xxxxxxxxxx>
> Signed-off-by: Jan Kara <jack@xxxxxxx>
> ---
> 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@xxxxxxx +49 911 74053 688
SUSE Software Solutions Germany GmbH, 90409 Nürnberg
GF: F. Imendörffer, HRB 36809 (AG Nürnberg)
