Re: [Qemu-devel] [PATCH v7 4/5] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT

[Peter Xu <peterx@redhat.com>]

On Thu, Jun 07, 2018 at 07:59:22PM +0800, Wei Wang wrote:
> On 06/07/2018 02:32 PM, Peter Xu wrote:
> > On Thu, Jun 07, 2018 at 01:24:29PM +0800, Wei Wang wrote:
> > > On 06/06/2018 07:02 PM, Peter Xu wrote:
> > > > On Wed, Jun 06, 2018 at 06:04:23PM +0800, Wei Wang wrote:
> > > > > On 06/06/2018 01:42 PM, Peter Xu wrote:
> > > > > > IMHO migration states do not suite here.  IMHO bitmap syncing is too
> > > > > > frequently an operation, especially at the end of a precopy migration.
> > > > > > If you really want to introduce some notifiers, I would prefer
> > > > > > something new rather than fiddling around with migration state.  E.g.,
> > > > > > maybe a new migration event notifiers, then introduce two new events
> > > > > > for both start/end of bitmap syncing.
> > > > > Please see if below aligns to what you meant:
> > > > > 
> > > > > MigrationState {
> > > > > ...
> > > > > + int ram_save_state;
> > > > > 
> > > > > }
> > > > > 
> > > > > typedef enum RamSaveState {
> > > > >       RAM_SAVE_BEGIN = 0,
> > > > >       RAM_SAVE_END = 1,
> > > > >       RAM_SAVE_MAX = 2
> > > > > }
> > > > > 
> > > > > then at the step 1) and 3) you concluded somewhere below, we change the
> > > > > state and invoke the callback.
> > > > I mean something like this:
> > > > 
> > > > 1693c64c27 ("postcopy: Add notifier chain", 2018-03-20)
> > > > 
> > > > That was a postcopy-only notifier.  Maybe we can generalize it into a
> > > > more common notifier for the migration framework so that we can even
> > > > register with non-postcopy events like bitmap syncing?
> > > Precopy already has its own notifiers: git 99a0db9b
> > > If we want to reuse, that one would be more suitable. I think mixing
> > > non-related events into one notifier list isn't nice.
> > I think that's only for migration state changes?
> > 
> > > > > Btw, the migration_state_notifiers is already there, but seems not really
> > > > > used (I only tracked spice-core.c called
> > > > > add_migration_state_change_notifier). I thought adding new migration states
> > > > > can reuse all that we have.
> > > > > What's your real concern about that? (not sure how defining new events would
> > > > > make a difference)
> > > > Migration state is exposed via control path (QMP).  Adding new states
> > > > mean that the QMP clients will see more.  IMO that's not really
> > > > anything that a QMP client will need to know, instead we can keep it
> > > > internally.  That's a reason from compatibility pov.
> > > > 
> > > > Meanwhile, it's not really a state-thing at all for me.  It looks
> > > > really more like hook or event (start/stop of sync).
> > > Thanks for sharing your concerns in detail, which are quite helpful for the
> > > discussion. To reuse 99a0db9b, we can also add sub-states (or say events),
> > > instead of new migration states.
> > > For example, we can still define "enum RamSaveState" as above, which can be
> > > an indication for the notifier queued on the 99a0db9b notider_list to decide
> > > whether to call start or stop.
> > > Does this solve your concern?
> > Frankly speaking I don't fully understand how you would add that
> > sub-state.  If you are confident with the idea, maybe you can post
> > your new version with the change, then I can read the code.
> 
> Sure. Code is more straightforward for this one. Let's check it in the new
> version.
> 
> > > > > > This is not that obvious to me.  For now I think it's true, since when
> > > > > > we call stop() we'll take the mutex, meanwhile the mutex is actually
> > > > > > always held by the iothread (in the big loop in
> > > > > > virtio_balloon_poll_free_page_hints) until either:
> > > > > > 
> > > > > > - it sleeps in qemu_cond_wait() [1], or
> > > > > > - it leaves the big loop [2]
> > > > > > 
> > > > > > Since I don't see anyone who will set dev->block_iothread to true for
> > > > > > the balloon device, then [1] cannot happen;
> > > > > there is a case in virtio_balloon_set_status which sets dev->block_iothread
> > > > > to true.
> > > > > 
> > > > > Did you mean the free_page_lock mutex? it is released at the bottom of the
> > > > > while() loop in virtio_balloon_poll_free_page_hint. It's actually released
> > > > > for every hint. That is,
> > > > > 
> > > > > while(1){
> > > > >       take the lock;
> > > > >       process 1 hint from the vq;
> > > > >       release the lock;
> > > > > }
> > > > Ah, so now I understand why you need the lock to be inside the loop,
> > > > since the loop is busy polling actually.  Is it possible to do this in
> > > > an async way?
> > > We need to use polling here because of some back story in the guest side
> > > (due to some locks being held) that makes it a barrier to sending
> > > notifications for each hints.
> > Any link to the "back story" that I can learn about? :) If it's too
> > complicated a problem and you think I don't need to understand at all,
> > please feel free to do so.
> 
> I searched a little bit, and forgot where we discussed this one. But the
> conclusion is that we don't want kick happens when the mm lock is held.
> Also, polling is a good idea here to me.
> There are 32 versions of kernel patch discussions scattered, interesting to
> watch, but might take too much time. Also people usually have different
> thoughts (sometimes with partial understanding) when they watch something
> (we even have many different versions of implementations ourselves if you
> check the whole 32 versions). It's not easy to get here with many consensus.
> That's why I hope our discussion could be more focused on the migration
> part, which is the last part that has not be fully finalized.

It's ok.

I'd be focused on migration part if you have a very clear interface
declared. :) You know, it was not even clear to me before I read the
series on whether the free_page_stop() operation is synchronous. And
IMHO that's really important even if I focus on migration review.

I'd say I'll treat reviewers somehow differently from you.  But I
don't think that worth a debate.

> 
> 
> 
> > Then I would assume at least Michael has
> > fully acknowledged that idea, and I can just stop putting more time on
> > this part.
> 
> Yes, he's been on the loop since the beginning.
> 
> 
> > 
> > Besides, if you are going to use a busy loop, then I would be not
> > quite sure about whether you really want to share that iothread with
> > others, since AFAIU that's not how iothread is designed (which is
> > mostly event-based and should not welcome out-of-control blocking in
> > the handler of events).  Though I am not 100% confident about my
> > understaning on that, I only raise this question up.  Anyway you'll
> > just take over the thread for a while without sharing, and after the
> > burst IOs it's mostly never used (until the next bitmap sync).  Then
> > it seems a bit confusing to me on why you need to share that after
> > all.
> 
> Not necessarily _need_ to share it, I meant it can be shared using qemu
> command line.
> Live migration doesn't happen all the time, and that optimization doesn't
> run that long, if users want to have other BHs run in this iothread context,
> they can only create one iothread via the qemu cmd line.

IMO iothreads and aiocontexts are for event-driven model.  Busy loop
is not an event-driven model.  Here if we want a busy loop I'll create
a thread when start page hinting, then join the thread when done.

But I'll stop commenting on this.  Please prepare a more clear
interface for migration in your next post.  I'll read that.

> 
> 
> > 
> > > > I'm a bit curious on how much time will it use to do
> > > > one round of the free page hints (e.g., an idle guest with 8G mem, or
> > > > any configuration you tested)?  I suppose during that time the
> > > > iothread will be held steady with 100% cpu usage, am I right?
> > > Compared to the time spent by the legacy migration to send free pages, that
> > > small amount of CPU usage spent on filtering free pages could be neglected.
> > > Grinding a chopper will not hold up the work of cutting firewood :)
> > Sorry I didn't express myself clearly.
> > 
> > My question was that, have you measured how long time it will take
> > from starting of the free page hints (when balloon state is set to
> > FREE_PAGE_REPORT_S_REQUESTED), until it completes (when QEMU receives
> > the VIRTIO_BALLOON_FREE_PAGE_REPORT_STOP_ID, then set the status to
> > FREE_PAGE_REPORT_S_STOP)?
> > 
> 
> I vaguely remember it's several ms (for around 7.5G free pages) long time
> ago. What would be the concern behind that number you want to know?

Because roughly I know the time between two bitmap syncs.  Then I will
know how possible a free page hinting process won't stop until the
next bitmap sync happens.

Thanks,

-- 
Peter Xu