Re: [PATCH 1/6] xen-blkfront: quiesce/unquiesce queue instead of start/stop queues

Ming Lei
2017-07-18T00:54:49Z

On Mon, Jul 17, 2017 at 05:02:27PM +0100, Roger Pau Monnï¿½ wrote:
> On Mon, Jul 17, 2017 at 11:06:28PM +0800, Ming Lei wrote:
> > On Mon, Jul 17, 2017 at 12:20:56PM +0100, Roger Pau Monnï¿½ wrote:
> > > On Sat, Jul 15, 2017 at 07:15:56AM +0800, Ming Lei wrote:
> > > > stopping queue may cause race and may not stop the queue really
> > > > after the API returns, and we have improved quiescing
> > > > interface and it really can block dispatching once it returns.
> > > >
> > > > So switch to quiesce/unquiece like what we did on other drivers
> > > > (NVMe, NBD, mtip32xx, ...)
> > > >
> > > > The blk_mq_stop_hw_queues() and blk_mq_start_stopped_hw_queues()
> > > > used in blkif_queue_rq() and blkif_interrupt() are for congestion
> > > > control, we leave it as it is since it is safe for this usage.
> > >
> > > Again I yet don't understand the difference between those two, neither
> > > why start/stop is not fixed instead of introducing quiesce/unquiece.
> >
> > There are two usages covered by start/stop now:
> >
> > - congestion control for handling queue busy(BLK_STS_RESOURCE), now
> > only xen-blkfront and virtio-blk use that
> >
> > - other usages, such as in xlvbd_release_gendisk(), for blocking
> > IO to driver/device
> >
> > For the 1st case, it is usually fine to use stop/start
> >
> > For the 2nd case, stop queue isn't enough, and we can't guarantee
> > no IO is dispatched to device/driver after returning from stop queue,
> > for details. Most of this usage have been fixed by Sagi Grimberg:
>
> OK, so basically after calling stop the queue might still be running.
>
> > We can't use quiesce/unquiesce for replacing stop/start in the
> > case of BLK_STS_RESOURCE, because quiesce may sleep, and we needn't
> > block IO for this usage actually.
>
> Do you mean that quiesce/unquiesce cannot be used while holding a
> spinlock?

Yes.

>
> >
> > > Not to mention that start/stop is not documented, which makes all this
> > > even more fun.
> >
> > Did you read comment of blk_mq_stop_hw_queue() and
> > blk_mq_stop_hw_queues() in linus tree?
>
> OK, this has been added very recently.
>
> > >
> > > Anyway I would like to ask, is the way to re-start a stopped queue the
> > > same way to unquiece?
> >
> > I don't know what your exact question, but it is definitely that
> > unquiesce is counter part of quiesce, and quiesce/unquiesce doesn't
> > depend on 'stopped' state any more if you take a look at the code.
> >
> > >
> > > If not I would rather prefer that start/stop or quiece/unquiece is
> > > used exclusively, in order to not make the code even more complex. It
> >
> > I do not think the code becomes more complex, please see the line change
> > of this patch:
>
> Before this patch blkfront used:
> blk_mq_stop_hw_queues
> blk_mq_start_stopped_hw_queues
> blk_mq_kick_requeue_list
>
> After the patch it uses:
> blk_mq_quiesce_queue
> blk_mq_unquiesce_queue
> blk_mq_stop_hw_queues
> blk_mq_start_stopped_hw_queues
> blk_mq_kick_requeue_list
> blk_mq_run_hw_queues
>
> It's not about line changes, but the amount of interfaces blkfront has
> to use. Apart from introducing the quiesce/unquiesce, you also
> introduce a call to blk_mq_run_hw_queues, which is not documented in
> the commit message.
>
> > > seems fairly easy to mess up and call "start" on a "quiesced" queue
> > > (or the other way around).
> >
> > Definitely it shouldn't be worried because start/stop is removed
> > in this patchset.
>
> Hm, how is that? I haven't seen any patch to blkfront to remove the
> usage of start/stop, am I missing something?

http://marc.info/?l=linux-block&m=150007418321196&w=2

--
Ming