On Mon 07-06-21 12:05:52, Hannes Reinecke wrote:
> 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() ...
I think we do check. blk_mq_get_driver_tag() has:
if (rq->tag == BLK_MQ_NO_TAG && !__blk_mq_get_driver_tag(rq))
return false;
if ((hctx->flags & BLK_MQ_F_TAG_QUEUE_SHARED) &&
!(rq->rq_flags & RQF_MQ_INFLIGHT)) {
rq->rq_flags |= RQF_MQ_INFLIGHT;
__blk_mq_inc_active_requests(hctx);
}
hctx->tags->rqs[rq->tag] = rq;
So once we call it, rq->tag will be != BLK_MQ_NO_TAG and RQF_MQ_INFLIGHT
will be set. So neither __blk_mq_get_driver_tag() nor
__blk_mq_inc_active_requests() will get repeated.
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
