Re: [PATCH 4/4] ublk: improve handling of saturated queues when ublk server exits

[Ming Lei]

On Wed, Mar 26, 2025 at 11:54:16AM -0600, Uday Shankar wrote:
> On Wed, Mar 26, 2025 at 01:38:35PM +0800, Ming Lei wrote:
> > On Tue, Mar 25, 2025 at 04:19:34PM -0600, Uday Shankar wrote:
> > > There are currently two ways in which ublk server exit is detected by
> > > ublk_drv:
> > > 
> > > 1. uring_cmd cancellation. If there are any outstanding uring_cmds which
> > >    have not been completed to the ublk server when it exits, io_uring
> > >    calls the uring_cmd callback with a special cancellation flag as the
> > >    issuing task is exiting.
> > > 2. I/O timeout. This is needed in addition to the above to handle the
> > >    "saturated queue" case, when all I/Os for a given queue are in the
> > >    ublk server, and therefore there are no outstanding uring_cmds to
> > >    cancel when the ublk server exits.
> > > 
> > > The second method detects ublk server exit only after a long delay
> > > (~30s, the default timeout assigned by the block layer). Any
> > > applications using the ublk device will be left hanging for these 30s
> > > before seeing an error/knowing anything went wrong. This problem is
> > > illustrated by running the new test_generic_02 against a ublk_drv which
> > > doesn't have the fix:
> > > 
> > > selftests: ublk: test_generic_02.sh
> > > dev id is 0
> > > dd: error writing '/dev/ublkb0': Input/output error
> > > 1+0 records in
> > > 0+0 records out
> > > 0 bytes copied, 30.0611 s, 0.0 kB/s
> > > DEAD
> > > dd took 31 seconds to exit (>= 5s tolerance)!
> > > generic_02 : [FAIL]
> > > 
> > > Fix this by instead handling the saturated queue case in the ublk
> > > character file release callback. This happens during ublk server exit
> > > and handles the issue much more quickly than an I/O timeout:
> > 
> > Another solution is to override default 30sec 'timeout'.
> 
> Yes, but that still will introduce unnecessary delays, since it is a
> polling-based solution (very similar to monitor_work we used to have).
> Also it will add complexity to the unprivileged case, since that
> actually cares about timeout and we will have to track the "real"
> timeout separately.
> 
> > 
> > > 
> > > selftests: ublk: test_generic_02.sh
> > > dev id is 0
> > > dd: error writing '/dev/ublkb0': Input/output error
> > > 1+0 records in
> > > 0+0 records out
> > > 0 bytes copied, 0.0376731 s, 0.0 kB/s
> > > DEAD
> > > generic_02 : [PASS]
> > > 
> > > Signed-off-by: Uday Shankar <ushankar@xxxxxxxxxxxxxxx>
> > > ---
> > >  drivers/block/ublk_drv.c                        | 40 +++++++++++------------
> > >  tools/testing/selftests/ublk/Makefile           |  1 +
> > >  tools/testing/selftests/ublk/kublk.c            |  3 ++
> > >  tools/testing/selftests/ublk/kublk.h            |  3 ++
> > >  tools/testing/selftests/ublk/null.c             |  4 +++
> > >  tools/testing/selftests/ublk/test_generic_02.sh | 43 +++++++++++++++++++++++++
> > >  6 files changed, 72 insertions(+), 22 deletions(-)
> > > 
> > > diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
> > > index c060da409ed8a888b7e414c9065efd2cbd6d57d7..1816b2cac01056dc9d01455759594af43c5f78d6 100644
> > > --- a/drivers/block/ublk_drv.c
> > > +++ b/drivers/block/ublk_drv.c
> > > @@ -1247,8 +1247,6 @@ static void ublk_queue_cmd(struct ublk_queue *ubq, struct request *rq)
> > >  static enum blk_eh_timer_return ublk_timeout(struct request *rq)
> > >  {
> > >  	struct ublk_queue *ubq = rq->mq_hctx->driver_data;
> > > -	unsigned int nr_inflight = 0;
> > > -	int i;
> > >  
> > >  	if (ubq->flags & UBLK_F_UNPRIVILEGED_DEV) {
> > >  		if (!ubq->timeout) {
> > > @@ -1259,26 +1257,6 @@ static enum blk_eh_timer_return ublk_timeout(struct request *rq)
> > >  		return BLK_EH_DONE;
> > >  	}
> > >  
> > > -	if (!ubq_daemon_is_dying(ubq))
> > > -		return BLK_EH_RESET_TIMER;
> > > -
> > > -	for (i = 0; i < ubq->q_depth; i++) {
> > > -		struct ublk_io *io = &ubq->ios[i];
> > > -
> > > -		if (!(io->flags & UBLK_IO_FLAG_ACTIVE))
> > > -			nr_inflight++;
> > > -	}
> > > -
> > > -	/* cancelable uring_cmd can't help us if all commands are in-flight */
> > > -	if (nr_inflight == ubq->q_depth) {
> > > -		struct ublk_device *ub = ubq->dev;
> > > -
> > > -		if (ublk_abort_requests(ub, ubq)) {
> > > -			schedule_work(&ub->nosrv_work);
> > > -		}
> > > -		return BLK_EH_DONE;
> > > -	}
> > > -
> > >  	return BLK_EH_RESET_TIMER;
> > >  }
> > >  
> > > @@ -1351,6 +1329,24 @@ static int ublk_ch_open(struct inode *inode, struct file *filp)
> > >  static int ublk_ch_release(struct inode *inode, struct file *filp)
> > >  {
> > >  	struct ublk_device *ub = filp->private_data;
> > > +	bool need_schedule = false;
> > > +	int i;
> > > +
> > > +	/*
> > > +	 * Error out any requests outstanding to the ublk server. This
> > > +	 * may have happened already (via uring_cmd cancellation), in
> > > +	 * which case it is not harmful to repeat. But uring_cmd
> > > +	 * cancellation does not handle queues which are fully saturated
> > > +	 * (all requests in ublk server), because from the kernel's POV,
> > > +	 * there are no outstanding uring_cmds to cancel. This code
> > > +	 * handles such queues.
> > > +	 */
> > > +
> > > +	for (i = 0; i < ub->dev_info.nr_hw_queues; i++)
> > > +		need_schedule |= ublk_abort_requests(ub, ublk_get_queue(ub, i));
> > > +
> > > +	if (need_schedule)
> > > +		schedule_work(&ub->nosrv_work);
> > 
> > ublk_abort_requests() should be called only in case of queue dying,
> > since ublk server may open & close the char device multiple times.
> 
> Sure that is technically possible, however is any real ublk server doing
> this? Seems like a strange thing to do, and seems reasonable for the
> driver to transition the device to the nosrv state (dead or recovery,
> depending on flags) when the char device is closed, since in this case,
> no one can be handling I/O anymore.

ublk server should be free to open & close the char device multiple times,
but you patch limits ublk server to open & close the char device just once.

The limit looks too strong...

> 
> In general I feel like char device close is a nice place to centralize
> the transition to the nosrv state. It has a few nice properties:
> - Because all file references are released at this point, we're
>   guaranteed that all file-related activity (uring_cmds, pread/pwrite)
>   is quiesced.
> - This one place can handle both saturated and unsaturated queues.
> - It is "event-driven," i.e. our callback gets called when a certain
>   condition is met, instead of having to poll for a condition (like the
>   old monitor_work, or the timeout now)
> - It looks like we can sleep in the char device close context, so we
>   could inline nosrv_work.

I agree all above, unless:

1) open() / close() need to be allowed multiple times

2) for dealing with 1), you may have to check if queue is dying, and this
way may have to use ->ubq_daemon, which is set when starting ublk, and cleared
when freeing ublk char. So race is added here, which need to be addressed.

> 
> This also is a step in the right direction IMO for resurrecting this old
> work to get rid of 1:1 ublk server thread to hctx restriction
> 
> https://lore.kernel.org/linux-block/20241002224437.3088981-1-ushankar@xxxxxxxxxxxxxxx/T/#u

That is definitely one good direction.

> 
> > For understanding if queue is dying, ->ubq_damon need to be checked,
> > however it may not be set yet and the current context is not same with
> > the ubq_daemon context, so I feel it is a bit fragile to bring queue
> > reference into ->release() callback.
> > 
> > Many libublksrv tests are failed with this patch or kernel panic, even
> > with the above check added:
> > 
> >         make test T=generic
> 
> Thanks, I will look at and address these failures.
> 
> Is there any plan to bring these tests into the new ublk selftests
> framework?

The two stress tests should be very similar with ublksrv's, just MQ isn't enabled.

I will enable them later.

The other big part is recovery test, which may take some time. I am a little busy
recently, it is great if anyone would like to pull recovery test in. Otherwise,
it may take a while.



thanks,
Ming