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

From: Roger Pau Monné
Date: Thu Jul 28 2016 - 03:49:41 EST


On Thu, Jul 28, 2016 at 07:05:05AM +0800, Bob Liu wrote:
>
> 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().

Right, stopping the queues doesn't seem to be an issue?

> * 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.

I'm afraid I don't know much about the new blk-mq API, you will have to ask
the blk-mq maintainers for solutions, since this was possible with the
previous API (and I would consider a regression that the new blk-mq API
doesn't allow it).

Roger.