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Re: [Qemu-devel] [PATCH 1/2] virtio-balloon: export all balloon statisti


From: Roman Kagan
Subject: Re: [Qemu-devel] [PATCH 1/2] virtio-balloon: export all balloon statistics
Date: Thu, 25 Feb 2016 14:17:38 +0300
User-agent: Mutt/1.5.24 (2015-08-30)

On Thu, Feb 25, 2016 at 11:46:59AM +0100, Markus Armbruster wrote:
> "Michael S. Tsirkin" <address@hidden> writes:
> 
> > On Thu, Feb 25, 2016 at 12:30:21PM +0300, Roman Kagan wrote:
> >> On Thu, Feb 25, 2016 at 10:54:17AM +0200, Michael S. Tsirkin wrote:
> >> > On Thu, Feb 25, 2016 at 09:44:06AM +0100, Markus Armbruster wrote:
> >> > > "Denis V. Lunev" <address@hidden> writes:
> >> > > 
> >> > > > On 02/24/2016 06:43 PM, Eric Blake wrote:
> >> > > >> On 02/24/2016 07:31 AM, Michael S. Tsirkin wrote:
> >> > > >>> Roman Kagan <address@hidden> writes:
> >> > > >>>> On Tue, Feb 23, 2016 at 05:49:21PM +0200, Michael S. Tsirkin 
> >> > > >>>> wrote:
> >> > > >>>>> On Tue, Feb 23, 2016 at 06:29:33PM +0300, Denis V. Lunev wrote:
> >> > > >>>>> > On 02/23/2016 06:24 PM, Michael S. Tsirkin wrote:
> >> > > >>>>> > >On Tue, Feb 23, 2016 at 05:59:44PM +0300, Denis V. Lunev 
> >> > > >>>>> > >wrote:
> >> > > >>>>> > >>From: Igor Redko <address@hidden>
> >> > > >>>>> > >>
> >> > > >>>>> > >>We are making experiments with different autoballooning 
> >> > > >>>>> > >>strategies
> >> > > >>>>> > >>based on the guest behavior. Thus we need to experiment with 
> >> > > >>>>> > >>different
> >> > > >>>>> > >>guest statistics. For now every counter change requires QEMU 
> >> > > >>>>> > >>recompilation
> >> > > >>>>> > >>and dances with Libvirt.
> >> > > >>>>> > >>
> >> > > >>>>> > >>This patch introduces transport for unrecognized counters in 
> >> > > >>>>> > >>virtio-balloon.
> >> > > >>>>> > >>This transport can be used for measuring benefits from using 
> >> > > >>>>> > >>new
> >> > > >>>>> > >>balloon counters, before submitting any patches. Current 
> >> > > >>>>> > >>alternative
> >> > > >>>>> > >>is 'guest-exec' transport which isn't made for such delicate 
> >> > > >>>>> > >>matters
> >> > > >>>>> > >>and can influence test results.
> >> > > >>>>> > >>
> >> > > >>>>> > >>Originally all counters with tag >= VIRTIO_BALLOON_S_NR were 
> >> > > >>>>> > >>ignored.
> >> > > >>>>> > >>Instead of this we keep first (VIRTIO_BALLOON_S_NR + 32) 
> >> > > >>>>> > >>counters from the
> >> > > >>>>> > >>queue and pass unrecognized ones with the following names: 
> >> > > >>>>> > >>'x-stat-XXXX',
> >> > > >>>>> > >>where XXXX is a tag number in hex. Defined counters are 
> >> > > >>>>> > >>reported with their
> >> > > >>>>> > >>regular names.
> >> > > >>>>> > >>
> >> > > >>>>> > >>Signed-off-by: Igor Redko <address@hidden>
> >> > > >>>>> > >>Signed-off-by: Denis V. Lunev <address@hidden>
> >> > > >>>>> > >>CC: Michael S. Tsirkin <address@hidden>
> >> > > >>>>> > >This seems to open the ABI to abuse.
> >> > > >>>>> > >Seems like a reasonable way to experiment though.
> >> > > >>>>> > >How about adding this within #if 0 statements?
> >> > > >>>>> > >You can uncomment them for debugging ...
> >> > > >>>>> > I'd prefer to have this enabled.
> >> > > 
> >> > > Yes, conditional compilation should be used sparingly.  I don't have an
> >> > > opinion on whether using it here is appropriate.
> >> > > 
> >> > > >>>>> > Why do you think that it opens "abuse" way?
> >> > > >>>>> 
> >> > > >>>>> Because people will use this to hack drivers and management tools
> >> > > >>>>> bypassing qemu.
> >> > > 
> >> > > Easy to avoid: shuffle the N in x-stat-N around from time to time, to
> >> > > reinforce the lesson that you must not rely on their presence or
> >> > > semantics.  I doubt it'll be necessary beyond the renumbering that
> >> > > happens naturally when we add supported counters, or the reshuffling
> >> > > that happens when somebody messes with the unsupported counters.
> >> > > 
> >> > > >>>> I'm curious why you think it's a problem?  Even the existing 
> >> > > >>>> stats are
> >> > > >>>> simply propagated to the management level by qemu with no 
> >> > > >>>> processing
> >> > > >>>> other than assigning text labels.  The proposed naming scheme for
> >> > > >>>> unrecognized counters includes "x-" prefix which explicitly marks 
> >> > > >>>> them
> >> > > >>>> as unstable so people using them take their risk.
> >> > > >>>>
> >> > > >>>> One of the benefits is forward compatibility, so that counters 
> >> > > >>>> that have
> >> > > >>>> graduated into supported ones and have got their own number and 
> >> > > >>>> name,
> >> > > >>>> can be made to work with qemu that doesn't yet recognize them.
> >> > > >>> Then management does start relying on the x- prefixed things,
> >> > > >>> and once it's used to that it's a slippery slope.
> >> > > >> Any management tool that relies on an x- prefix name is broken.
> >> > > 
> >> > > Or at least assumes the full risk of breaking without notice whenever
> >> > > QEMU changes.  Abbreviating that to just "broken" seems fair enough :)
> >> > > 
> >> > > >>                                                                  
> >> > > >> We've
> >> > > >> explicitly documented that the x- prefix is unstable and liable to 
> >> > > >> go
> >> > > >> away with a future release. Any management app that wants to use a
> >> > > >> feature beginning with x- should FIRST push hard to get the x- 
> >> > > >> removed
> >> > > >> and stabilize the interface (and libvirt, at least, does just that).
> >> > > >>
> >> > > > this was exactly an original idea. Names started with 'x-' are
> >> > > > _officially_ unstable and for debug purpose. That is why I'd
> >> > > > prefer if v2 of the patchset will be taken.
> >> > > 
> >> > > Looks like fair use of x- to me.
> >> > 
> >> > 
> >> > Well I already heard:
> >> > 
> >> >  One of the benefits is forward compatibility, so that counters that have
> >> >  graduated into supported ones and have got their own number and name,
> >> >  can be made to work with qemu that doesn't yet recognize them.
> >> > 
> >> > in this thread, which seems to mean exactly that people start planning 
> >> > to abuse it
> >> > even before it's merged.
> >> 
> >> That quote (from yours truly) states the opposite.
> >> 
> >> The whole point is that there are several participants in the process,
> >> with independent release cycles and policies, but with a common
> >> "registry" of supported stats (with the master copy being in the kernel,
> >> right?).
> >
> > For most devices it's the virtio spec.
> >
> >> Once a counter is accepted there, you can start shipping the
> >> guest driver providing it, and you don't have to wait until qemu catches
> >> up: your management level can read "x-stat-NEW_NUMBER" *or* "new_name",
> >> as both NEW_NUMBER and new_name are now allocated for that new counter.
> >> 
> >> So yes, people are planning to use it, in particluar, before it's merged
> >> into qemu proper, but I don't see how that creates any extra maintenance
> >> burden on qemu side.  Anybody using x- is on their own; the scheme I
> >> sketched seems reasonably safe but is the headache of that management
> >> software anyway.
> >> 
> >> Roman.
> >
> > Basically if you do this hack QEMU must not reuse the x-stat-NEW_NUMBER
> > ever, otherwise management handling it will intepret it
> > as legacy QEMU and will break.
> 
> No, QEMU should aggressively reuse the number part.  Heck, it's free to
> randomly mess with it without notice.  Makes the x-stat-N effectively
> useless for anything but experimenting.  Which is exactly the point of
> naming them x-.

I must be missing something...  QEMU has no business with the number at
all unless it recognizes it; in that case it only replaces the dumb
x-stat-N label with the one it knows.  How can it "randomly mess with
it"?

Let's get this straight: the only thing QEMU does with balloon stats is
marshalling them into json.  (For that matter, libvirt only unmarshalls
them back into an array of (int tag, long value) pairs so is similar.)
Basically QEMU only plays the role of transport for memory stats between
the guest driver and the management layer; I'm not even sure why it has
to know what the individual fields mean.  What this patch proposes is
essentially to make QEMU not stand in the way of the data it transports;
it's only the endpoints' responsibility to agree on the interpretation
of the contents.

Roman.



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