Re: [PATCH 3/3] xen-blkfront: dynamic configuration of per-vbd resources

["Roger Pau Monné" <roger.pau@citrix.com>]

On Sat, Jul 23, 2016 at 06:18:23AM +0800, Bob Liu wrote:
> 
> On 07/22/2016 07:45 PM, Roger Pau Monné wrote:
> > On Fri, Jul 22, 2016 at 05:43:32PM +0800, Bob Liu wrote:
> >>
> >> On 07/22/2016 05:34 PM, Roger Pau Monné wrote:
> >>> On Fri, Jul 22, 2016 at 04:17:48PM +0800, Bob Liu wrote:
> >>>>
> >>>> On 07/22/2016 03:45 PM, Roger Pau Monné wrote:
> >>>>> On Thu, Jul 21, 2016 at 06:08:05PM +0800, Bob Liu wrote:
> >>>>>>
> >>>>>> On 07/21/2016 04:57 PM, Roger Pau Monné wrote:
> >>>> ..[snip]..
> >>>>>>>> +
> >>>>>>>> +static ssize_t dynamic_reconfig_device(struct blkfront_info *info, ssize_t count)
> >>>>>>>> +{
> >>>>>>>> +	unsigned int i;
> >>>>>>>> +	int err = -EBUSY;
> >>>>>>>> +
> >>>>>>>> +	/*
> >>>>>>>> +	 * Make sure no migration in parallel, device lock is actually a
> >>>>>>>> +	 * mutex.
> >>>>>>>> +	 */
> >>>>>>>> +	if (!device_trylock(&info->xbdev->dev)) {
> >>>>>>>> +		pr_err("Fail to acquire dev:%s lock, may be in migration.\n",
> >>>>>>>> +			dev_name(&info->xbdev->dev));
> >>>>>>>> +		return err;
> >>>>>>>> +	}
> >>>>>>>> +
> >>>>>>>> +	/*
> >>>>>>>> +	 * Prevent new requests and guarantee no uncompleted reqs.
> >>>>>>>> +	 */
> >>>>>>>> +	blk_mq_freeze_queue(info->rq);
> >>>>>>>> +	if (part_in_flight(&info->gd->part0))
> >>>>>>>> +		goto out;
> >>>>>>>> +
> >>>>>>>> +	/*
> >>>>>>>> +	 * Front 				Backend
> >>>>>>>> +	 * Switch to XenbusStateClosed
> >>>>>>>> +	 *					frontend_changed():
> >>>>>>>> +	 *					 case XenbusStateClosed:
> >>>>>>>> +	 *						xen_blkif_disconnect()
> >>>>>>>> +	 *						Switch to XenbusStateClosed
> >>>>>>>> +	 * blkfront_resume():
> >>>>>>>> +	 *					frontend_changed():
> >>>>>>>> +	 *						reconnect
> >>>>>>>> +	 * Wait until XenbusStateConnected
> >>>>>>>> +	 */
> >>>>>>>> +	info->reconfiguring = true;
> >>>>>>>> +	xenbus_switch_state(info->xbdev, XenbusStateClosed);
> >>>>>>>> +
> >>>>>>>> +	/* Poll every 100ms, 1 minute timeout. */
> >>>>>>>> +	for (i = 0; i < 600; i++) {
> >>>>>>>> +		/*
> >>>>>>>> +		 * Wait backend enter XenbusStateClosed, blkback_changed()
> >>>>>>>> +		 * will clear reconfiguring.
> >>>>>>>> +		 */
> >>>>>>>> +		if (!info->reconfiguring)
> >>>>>>>> +			goto resume;
> >>>>>>>> +		schedule_timeout_interruptible(msecs_to_jiffies(100));
> >>>>>>>> +	}
> >>>>>>>
> >>>>>>> Instead of having this wait, could you just set info->reconfiguring = 1, set 
> >>>>>>> the frontend state to XenbusStateClosed and mimic exactly what a resume from 
> >>>>>>> suspension does? blkback_changed would have to set the frontend state to 
> >>>>>>> InitWait when it detects that the backend has switched to Closed, and call 
> >>>>>>> blkfront_resume.
> >>>>>>
> >>>>>>
> >>>>>> I think that won't work.
> >>>>>> In the real "resume" case, the power management system will trigger all ->resume() path.
> >>>>>> But there is no place for dynamic configuration.
> >>>>>
> >>>>> Hello,
> >>>>>
> >>>>> I think it should be possible to set info->reconfiguring and wait for the 
> >>>>> backend to switch to state Closed, at that point we should call blkif_resume 
> >>>>> (from blkback_changed) and the backend will follow the reconection.
> >>>>>
> >>>>
> >>>> Okay, I get your point. Yes, that's an option.
> >>>>
> >>>> But this will make 'dynamic configuration' to be async, I'm worry about the end-user will get panic.
> >>>> E.g
> >>>> A end-user "echo <new value> > /sys/devices/vbd-xxx/max_indirect_segs",
> >>>> but then the device will be Closed and disappeared, the user have to wait for a random time so that the device can resume.
> >>>
> >>> That should not happen, AFAICT on migration the device never dissapears. 
> >>
> >> Oh, yes.
> >>
> >>> alloc_disk and friends should not be called on resume from migration (see 
> >>> the switch in blkfront_connect, you should take the BLKIF_STATE_SUSPENDED 
> >>> path for the reconfiguration).
> >>>
> >>
> >> What about if the end-user starts I/O immediately after writing new value to /sys?
> >> But the resume is still in progress.
> > 
> > blkif_free already stops the queues by calling blk_mq_stop_hw_queues, and 
> > blkif_queue_request will refuse to put anything on the ring if the state 
> > is different than connected, which in turn makes blkif_queue_rq call 
> > blk_mq_stop_hw_queue to stop the queue, so it should be safe.
> > 
> 
> But this will surprise the end-user, our user script is like this:
> 1) echo <new value> > /sys/xxxx
> 2) Start I/O immediately.
> 	^^^ Fail because requests would be refused(even software queue was still freezed).

Which error do you get? AFAICT reads/writes should either block when the 
queue is full, or if the fd is in non-blocking mode it should return EAGAIN 
(which a properly coded application should be prepared to deal with 
gracefully if it's using non-blocking fds anyway).
 
> It's not good for the end user have to wait for a random time before restart I/O.
> 
> 
> There are two more concerns I have:
>  * blkif_resume() may fail, how the end-user can aware that if "echo <new value> > /sys/xxx" already returned success.

If you really think this is needed, can't you use a monitor or some kind of 
condition with a timeout instead of open-coding it? Although I'm still not 
convinced that blocking here is TRTTD.

>  * We get the device lock and blk_mq_freeze_queue() in dynamic_reconfig_device(),
>    and then have to release in blkif_recover() asynchronously which makes the code more difficult to readable.

I'm not sure I'm following here, do you mean that you will pick the lock in 
dynamic_reconfig_device and release it in blkif_recover? Why wouldn't you 
release the lock in dynamic_reconfig_device after doing whatever is needed?

> As a result, I still prefer current synchronize way so that we can know whether blkif_resume fails,
> not break the end-user and more straightforward code.
> -- 
> Regards,
> -Bob