On Wed 02-06-21 17:25:52, 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.
> >
> > 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;
> > + } 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@xxxxxxxxxx>
OK, thanks for your review!
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
