Re: [PATCH v3] xen-blkfront: dynamic configuration of per-vbd resources

[Bob Liu <bob.liu@oracle.com>]

On 07/27/2016 10:24 PM, Roger Pau Monné wrote:
> On Wed, Jul 27, 2016 at 07:21:05PM +0800, Bob Liu wrote:
>>
>> On 07/27/2016 06:59 PM, Roger Pau Monné wrote:
>>> On Wed, Jul 27, 2016 at 11:21:25AM +0800, Bob Liu wrote:
>>> [...]
>>>> +static ssize_t dynamic_reconfig_device(struct blkfront_info *info, ssize_t count)
>>>> +{
>>>> +	/*
>>>> +	 * Prevent new requests even to software request queue.
>>>> +	 */
>>>> +	blk_mq_freeze_queue(info->rq);
>>>> +
>>>> +	/*
>>>> +	 * Guarantee no uncompleted reqs.
>>>> +	 */
>>>
>>> I'm also wondering, why do you need to guarantee that there are no 
>>> uncompleted requests? The resume procedure is going to call blkif_recover 
>>> that will take care of requeuing any unfinished requests that are on the 
>>> ring.
>>>
>>
>> Because there may have requests in the software request queue with more segments than
>> we can handle(if info->max_indirect_segments is reduced).
>>
>> The blkif_recover() can't handle this since blk-mq was introduced,
>> because there is no way to iterate the sw-request queues(blk_fetch_request() can't be used by blk-mq).
>>
>> So there is a bug in blkif_recover(), I was thinking implement the suspend function of blkfront_driver like:
> 
> Hm, this is a regression and should be fixed ASAP. I'm still not sure I 
> follow, don't blk_queue_max_segments change the number of segments the 
> requests on the queue are going to have? So that you will only have to 
> re-queue the requests already on the ring?
> 

That's not enough, request queues were split to software queues and hardware queues since blk-mq was introduced.
We need to consider two more things:
 * Stop new requests be added to software queues before blk_queue_max_segments() is called(still using old 'max-indirect-segments').
   I didn't see other way except call blk_mq_freeze_queue().

 * Requests already in software queues but with old 'max-indirect-segments' also have to be re-queued based on new 'max-indirect-segments'.
   Neither blk-mq API can do this.

> Waiting for the whole queue to be flushed before suspending is IMHO not 
> acceptable, it introduces an unbounded delay during migration if the backend 
> is slow for some reason.
> 

Right, I also hope there is better solution.

-- 
Regards,
-Bob