Re: [Qemu-devel] [libvirt] [PATCH 1/2] numa: deprecate 'mem' parameter of '-numa node' option

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Michal Privoznik (mprivozn@redhat.com) wrote:
> On 3/4/19 3:24 PM, Daniel P. Berrangé wrote:
> > On Mon, Mar 04, 2019 at 03:16:41PM +0100, Igor Mammedov wrote:
> > > On Mon, 4 Mar 2019 12:39:08 +0000
> > > Daniel P. Berrangé <berrange@redhat.com> wrote:
> > > 
> > > > On Mon, Mar 04, 2019 at 01:25:07PM +0100, Igor Mammedov wrote:
> > > > > On Mon, 04 Mar 2019 08:13:53 +0100
> > > > > Markus Armbruster <armbru@redhat.com> wrote:
> > > > > > Daniel P. Berrangé <berrange@redhat.com> writes:
> > > > > > > On Fri, Mar 01, 2019 at 06:33:28PM +0100, Igor Mammedov wrote:
> > > > > > > > On Fri, 1 Mar 2019 15:49:47 +0000
> > > > > > > > Daniel P. Berrangé <berrange@redhat.com> wrote:
> > > > > > > > > On Fri, Mar 01, 2019 at 04:42:15PM +0100, Igor Mammedov wrote:
> > > > > > > > > > The parameter allows to configure fake NUMA topology where guest
> > > > > > > > > > VM simulates NUMA topology but not actually getting a performance
> > > > > > > > > > benefits from it. The same or better results could be achieved
> > > > > > > > > > using 'memdev' parameter. In light of that any VM that uses NUMA
> > > > > > > > > > to get its benefits should use 'memdev' and to allow transition
> > > > > > > > > > initial RAM to device based model, deprecate 'mem' parameter as
> > > > > > > > > > its ad-hoc partitioning of initial RAM MemoryRegion can't be
> > > > > > > > > > translated to memdev based backend transparently to users and in
> > > > > > > > > > compatible manner (migration wise).
> > > > > > > > > > 
> > > > > > > > > > That will also allow to clean up a bit our numa code, leaving only
> > > > > > > > > > 'memdev' impl. in place and several boards that use node_mem
> > > > > > > > > > to generate FDT/ACPI description from it.
> > > > > > > > > 
> > > > > > > > > Can you confirm that the  'mem' and 'memdev' parameters to -numa
> > > > > > > > > are 100% live migration compatible in both directions ?  Libvirt
> > > > > > > > > would need this to be the case in order to use the 'memdev' syntax
> > > > > > > > > instead.
> > > > > > > > Unfortunately they are not migration compatible in any direction,
> > > > > > > > if it where possible to translate them to each other I'd alias 'mem'
> > > > > > > > to 'memdev' without deprecation. The former sends over only one
> > > > > > > > MemoryRegion to target, while the later sends over several (one per
> > > > > > > > memdev).
> > > > > > > 
> > > > > > > If we can't migration from one to the other, then we can not deprecate
> > > > > > > the existing 'mem' syntax. Even if libvirt were to provide a config
> > > > > > > option to let apps opt-in to the new syntax, we need to be able to
> > > > > > > support live migration of existing running VMs indefinitely. Effectively
> > > > > > > this means we need the to keep 'mem' support forever, or at least such
> > > > > > > a long time that it effectively means forever.
> > > > > > > 
> > > > > > > So I think this patch has to be dropped & replaced with one that
> > > > > > > simply documents that memdev syntax is preferred.
> > > > > > 
> > > > > > We have this habit of postulating absolutes like "can not deprecate"
> > > > > > instead of engaging with the tradeoffs.  We need to kick it.
> > > > > > 
> > > > > > So let's have an actual look at the tradeoffs.
> > > > > > 
> > > > > > We don't actually "support live migration of existing running VMs
> > > > > > indefinitely".
> > > > > > 
> > > > > > We support live migration to any newer version of QEMU that still
> > > > > > supports the machine type.
> > > > > > 
> > > > > > We support live migration to any older version of QEMU that already
> > > > > > supports the machine type and all the devices the machine uses.
> > > > > > 
> > > > > > Aside: "support" is really an honest best effort here.  If you rely on
> > > > > > it, use a downstream that puts in the (substantial!) QA work real
> > > > > > support takes.
> > > > > > 
> > > > > > Feature deprecation is not a contract to drop the feature after two
> > > > > > releases, or even five.  It's a formal notice that users of the feature
> > > > > > should transition to its replacement in an orderly manner.
> > > > > > 
> > > > > > If I understand Igor correctly, all users should transition away from
> > > > > > outdated NUMA configurations at least for new VMs in an orderly manner.
> > > > > Yes, we can postpone removing options until there are machines type
> > > > > versions that were capable to use it (unfortunate but probably
> > > > > unavoidable unless there is a migration trick to make transition
> > > > > transparent) but that should not stop us from disabling broken
> > > > > options on new machine types at least.
> > > > > 
> > > > > This series can serve as formal notice with follow up disabling of
> > > > > deprecated options for new machine types. (As Thomas noted, just warnings
> > > > > do not work and users continue to use broken features regardless whether
> > > > > they are don't know about issues or aware of it [*])
> > > > > 
> > > > > Hence suggested deprecation approach and enforced rejection of legacy
> > > > > numa options for new machine types in 2 releases so users would stop
> > > > > using them eventually.
> > > > 
> > > > When we deprecate something, we need to have a way for apps to use the
> > > > new alternative approach *at the same time*.  So even if we only want to
> > > > deprecate for new machine types, we still have to first solve the problem
> > > > of how mgmt apps will introspect QEMU to learn which machine types expect
> > > > the new options.
> > > I'm not aware any mechanism to introspect machine type options (existing
> > > or something being developed). Are/were there any ideas about it that were
> > > discussed in the past?
> > > 
> > > Aside from developing a new mechanism what are alternative approaches?
> > > I mean when we delete deprecated CLI option, how it's solved on libvirt
> > > side currently?
> > > 
> > > For example I don't see anything introspection related when we have been
> > > removing deprecated options recently.
> > 
> > Right, with other stuff we deprecate we've had a simpler time, as it
> > either didn't affect migration at all, or the new replacement stuff
> > was fully compatible with the migration data stream. IOW, libvirt
> > could unconditionally use the new feature as soon as it saw that it
> > exists in QEMU. We didn't have any machine type dependancy to deal
> > with before now.
> 
> We couldn't have done that. How we would migrate from older qemu?
> 
> Anyway, now that I look into this (esp. git log) I came accross:
> 
> commit f309db1f4d51009bad0d32e12efc75530b66836b
> Author:     Michal Privoznik <mprivozn@redhat.com>
> AuthorDate: Thu Dec 18 12:36:48 2014 +0100
> Commit:     Michal Privoznik <mprivozn@redhat.com>
> CommitDate: Fri Dec 19 07:44:44 2014 +0100
> 
>     qemu: Create memory-backend-{ram,file} iff needed
> 
> Or this 7832fac84741d65e851dbdbfaf474785cbfdcf3c. We did try to generated
> newer cmd line but then for various reasong (e.g. avoiding triggering a qemu
> bug) we turned it off and make libvirt default to older (now deprecated) cmd
> line.
> 
> Frankly, I don't know how to proceed. Unless qemu is fixed to allow
> migration from deprecated to new cmd line (unlikely, if not impossible,
> right?) then I guess the only approach we can have is that:
> 
> 1) whenever so called cold booting a new machine (fresh, brand new start of
> a new domain) libvirt would default to modern cmd line,
>
> 2) on migration, libvirt would record in the migration stream (or status XML
> or wherever) that modern cmd line was generated and thus it'll make the
> destination generate modern cmd line too.
> 
> This solution still suffers a couple of problems:
> a) migration to older libvirt will fail as older libvirt won't recognize the
> flag set in 2) and therefore would default to deprecated cmd line
> b) migrating from one host to another won't modernize the cmd line
> 
> But I guess we have to draw a line somewhere (if we are not willing to write
> those migration patches).

What's interesting here is that this problem isn't really machine-type
related; so providing introspection on the machine type doesn't
immediately help.  What we're actually trying to do here is (mis)use a
machine type as a proxy for knowing that both sides are new enough to
handle the new command line.

That's an OK thing to do, and if we did have introspection we could
add a fudge flag to say it's allowed now.

Dave


> Michal
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK