Re: [PATCH v2] block: Do not pull requests from the scheduler when we cannot dispatch them

From: Jan Kara <jack@suse.cz>
To: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>,
	linux-block@vger.kernel.org, Ming Lei <ming.lei@redhat.com>,
	Paolo Valente <paolo.valente@linaro.org>
Subject: Re: [PATCH v2] block: Do not pull requests from the scheduler when we cannot dispatch them
Date: Wed, 16 Jun 2021 17:51:01 +0200	[thread overview]
Message-ID: <20210616155101.GB28250@quack2.suse.cz> (raw)
In-Reply-To: <d61be138-7fc2-76d6-d01f-6ec555e46f29@kernel.dk>

On Wed 16-06-21 09:43:06, Jens Axboe wrote:
> On 6/16/21 9:40 AM, Jan Kara wrote:
> > On Thu 03-06-21 12:47:21, 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@redhat.com>
> >> 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(-)
> >>
> >> Jens, can you please merge the patch? Thanks!
> > 
> > Ping Jens?
> 
> Didn't I reply? It's merged for a while:

No, I didn't get email back and didn't notice the patch is already in your
tree. Sorry for the noise.

								Honza

> 
> https://git.kernel.dk/cgit/linux-block/commit/?h=for-5.14/block&id=613471549f366cdf4170b81ce0f99f3867ec4d16
> 
> -- 
> Jens Axboe
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR