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

[Bob Liu]

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).

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.
 
 * 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.

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