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

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

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.

Roger.