On Fri 21-05-21 09:29:33, Ming Lei wrote:
> 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?
No. blk_mq_update_dispatch_busy() has:
if (hctx->queue->elevator)
return;
> > 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@xxxxxxx>
> > ---
> > 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?
Ah yes, I forgot to mention that. I actually had nr_requests set to 1024 as
otherwise all requests get sucked into virtio-blk and no IO scheduling
happens either.
> 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?
I'll try it with modifying blk_mq_update_dispatch_busy() to be updated when
elevator is in use...
Honza
>
>
> 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
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
