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From: Jan Kara <jack@suse.cz>
To: Ming Lei <ming.lei@redhat.com>
Cc: Jan Kara <jack@suse.cz>, Jens Axboe <axboe@kernel.dk>,
	Paolo Valente <paolo.valente@linaro.org>,
	linux-block@vger.kernel.org
Subject: Re: [PATCH] block: Do not pull requests from the scheduler when we cannot dispatch them
Date: Thu, 3 Jun 2021 12:45:18 +0200	[thread overview]
Message-ID: <20210603104518.GJ23647@quack2.suse.cz> (raw)
In-Reply-To: <YLdOoFBz3k+ipxkC@T590>

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@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(-)
> > 
> > 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@redhat.com>

OK, thanks for your review!

									Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

      reply	other threads:[~2021-06-03 10:45 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-20 11:25 [PATCH] block: Do not pull requests from the scheduler when we cannot dispatch them Jan Kara
2021-05-21  1:29 ` Ming Lei
2021-05-21 11:20   ` Jan Kara
2021-05-21 11:27     ` Jan Kara
2021-05-21 13:18     ` Ming Lei
2021-06-02  9:25 ` Ming Lei
2021-06-03 10:45   ` Jan Kara [this message]

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