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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Mar 04, 2019 at 08:13:53AM +0100, Markus Armbruster 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.

If upstream deletes the feature, then that in turn breaks the downstream
unless downstream reverts the upstream change. When we have large overlap
between downstream & upstream maintainer, it is not beneficial to delete
the feature upstream as any effort saved upstream usually expands into
larger effort downstream.

> 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.
> 
> So, how could this formal notice be served constructively?
> 
> If we reject outdated NUMA configurations starting with machine type T,
> we can remove the means to create those configurations along with
> machine type T-1.  Won't happen anytime soon, will happen eventually,
> because in the long run, all machine types are dead (apologies to
> Keynes).
> 
> If we deprecate outdated NUMA configurations now, we can start rejecting
> them with new machine types after a suitable grace period.

How is libvirt going to know what machines it can use with the feature ?
We don't have any way to introspect machine type specific logic, since we
run all probing with "-machine none", and QEMU can't report anything about
machines without instantiating them.

Regards,
Daniel
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