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Re: [Qemu-devel] [RFC] QMP: add query-hotpluggable-cpus
From: |
David Gibson |
Subject: |
Re: [Qemu-devel] [RFC] QMP: add query-hotpluggable-cpus |
Date: |
Fri, 26 Feb 2016 15:12:26 +1100 |
User-agent: |
Mutt/1.5.24 (2015-08-30) |
On Thu, Feb 25, 2016 at 01:43:05PM +0100, Igor Mammedov wrote:
> On Thu, 25 Feb 2016 12:25:43 +1100
> David Gibson <address@hidden> wrote:
>
> > On Wed, Feb 24, 2016 at 03:17:54PM +0100, Igor Mammedov wrote:
> > > On Wed, 24 Feb 2016 12:54:17 +1100
> > > David Gibson <address@hidden> wrote:
> > >
> > > > On Tue, Feb 23, 2016 at 10:46:45AM +0100, Igor Mammedov wrote:
> > > > > On Mon, 22 Feb 2016 13:54:32 +1100
> > > > > David Gibson <address@hidden> wrote:
> > > > >
> > > > > > On Fri, Feb 19, 2016 at 04:49:11PM +0100, Igor Mammedov wrote:
> > > > > > > On Fri, 19 Feb 2016 15:38:48 +1100
> > > > > > > David Gibson <address@hidden> wrote:
> > > > > > >
> > > > > > > CCing thread a couple of libvirt guys.
> > > > > > >
> > > > > > > > On Thu, Feb 18, 2016 at 11:37:39AM +0100, Igor Mammedov wrote:
> > > > > > > >
> > > > > > > > > On Thu, 18 Feb 2016 14:39:52 +1100
> > > > > > > > > David Gibson <address@hidden> wrote:
> > > > > > > > >
> > > > > > > > > > On Tue, Feb 16, 2016 at 11:36:55AM +0100, Igor Mammedov
> > > > > > > > > > wrote:
> > > > > > > > > > > On Mon, 15 Feb 2016 20:43:41 +0100
> > > > > > > > > > > Markus Armbruster <address@hidden> wrote:
> > > > > > > > > > >
> > > > > > > > > > > > Igor Mammedov <address@hidden> writes:
> > > > > > > > > > > >
> > > > > > > > > > > > > it will allow mgmt to query present and possible to
> > > > > > > > > > > > > hotplug CPUs
> > > > > > > > > > > > > it is required from a target platform that wish to
> > > > > > > > > > > > > support
> > > > > > > > > > > > > command to set board specific
> > > > > > > > > > > > > MachineClass.possible_cpus() hook,
> > > > > > > > > > > > > which will return a list of possible CPUs with options
> > > > > > > > > > > > > that would be needed for hotplugging possible CPUs.
> > > > > > > > > > > > >
> > > > > > > > > > > > > For RFC there are:
> > > > > > > > > > > > > 'arch_id': 'int' - mandatory unique CPU number,
> > > > > > > > > > > > > for x86 it's APIC ID for ARM
> > > > > > > > > > > > > it's MPIDR
> > > > > > > > > > > > > 'type': 'str' - CPU object type for usage with
> > > > > > > > > > > > > device_add
> > > > > > > > > > > > >
> > > > > > > > > > > > > and a set of optional fields that would allows mgmt
> > > > > > > > > > > > > tools
> > > > > > > > > > > > > to know at what granularity and where a new CPU could
> > > > > > > > > > > > > be
> > > > > > > > > > > > > hotplugged;
> > > > > > > > > > > > > [node],[socket],[core],[thread]
> > > > > > > > > > > > > Hopefully that should cover needs for CPU hotplug
> > > > > > > > > > > > > porposes for
> > > > > > > > > > > > > magor targets and we can extend structure in future
> > > > > > > > > > > > > adding
> > > > > > > > > > > > > more fields if it will be needed.
> > > > > > > > > > > > >
> > > > > > > > > > > > > also for present CPUs there is a 'cpu_link' field
> > > > > > > > > > > > > which
> > > > > > > > > > > > > would allow mgmt inspect whatever object/abstraction
> > > > > > > > > > > > > the target platform considers as CPU object.
> > > > > > > > > > > > >
> > > > > > > > > > > > > For RFC purposes implements only for x86 target so
> > > > > > > > > > > > > far.
> > > > > > > > > > > >
> > > > > > > > > > > > Adding ad hoc queries as we go won't scale. Could this
> > > > > > > > > > > > be solved by a
> > > > > > > > > > > > generic introspection interface?
> > > > > > > > > > > Do you mean generic QOM introspection?
> > > > > > > > > > >
> > > > > > > > > > > Using QOM we could have '/cpus' container and create QOM
> > > > > > > > > > > links
> > > > > > > > > > > for exiting (populated links) and possible (empty links)
> > > > > > > > > > > CPUs.
> > > > > > > > > > > However in that case link's name will need have a special
> > > > > > > > > > > format
> > > > > > > > > > > that will convey an information necessary for mgmt to
> > > > > > > > > > > hotplug
> > > > > > > > > > > a CPU object, at least:
> > > > > > > > > > > - where: [node],[socket],[core],[thread] options
> > > > > > > > > > > - optionally what CPU object to use with device_add
> > > > > > > > > > > command
> > > > > > > > > >
> > > > > > > > > > Hmm.. is it not enough to follow the link and get the
> > > > > > > > > > topology
> > > > > > > > > > information by examining the target?
> > > > > > > > > One can't follow a link if it's an empty one, hence
> > > > > > > > > CPU placement information should be provided somehow,
> > > > > > > > > either:
> > > > > > > >
> > > > > > > > Ah, right, so the issue is determining the socket/core/thread
> > > > > > > > addresses that cpus which aren't yet present will have.
> > > > > > > >
> > > > > > > > > * by precreating cpu-package objects with properties that
> > > > > > > > > would describe it /could be inspected via OQM/
> > > > > > > >
> > > > > > > > So, we could do this, but I think the natural way would be to
> > > > > > > > have the
> > > > > > > > information for each potential thread in the package. Just
> > > > > > > > putting
> > > > > > > > say "core number" in the package itself assumes more than I'd
> > > > > > > > like
> > > > > > > > about how packages sit in the heirarchy. Plus, it means that
> > > > > > > > management has a bunch of cases to deal with: package has all
> > > > > > > > the
> > > > > > > > information, package has just a core id, package has just a
> > > > > > > > socket id,
> > > > > > > > and so forth.
> > > > > > > >
> > > > > > > > It is a but clunky that when the package is plugged, this
> > > > > > > > information
> > > > > > > > will have to sit parallel to the array of actual thread links.
> > > > > > > >
> > > > > > > > Markus or Andreas is there a natural way to present a list of
> > > > > > > > (node,
> > > > > > > > socket, core, thread) tuples in the package object? Preferably
> > > > > > > > without having to create a whole bunch of "potential thread"
> > > > > > > > objects
> > > > > > > > just for the purpose.
> > > > > > > I'm sorry but I couldn't parse above 2 paragraphs. The way I see
> > > > > > > whatever placement info QEMU will provide to mgmt, mgmt will have
> > > > > > > to deal with it in one way or another.
> > > > > > > Perhaps rephrasing and adding some examples might help to explain
> > > > > > > suggestion a bit better?
> > > > > >
> > > > > > Ok, so what I'm saying is that I think describing a location for the
> > > > > > package itself could be problematic. For some cases it will be ok,
> > > > > > but depending on exactly what the package represents on a particular
> > > > > > platform there could be a lot of options for how to represent it.
> > > > > >
> > > > > > What I'm suggesting instead is that instead of giving a location for
> > > > > > itself, the package lists the locations of all the threads it will
> > > > > > contain when it is enabled/present/whatever. Those locations can be
> > > > > > given as node/socket/core/thread tuples - which are properties that
> > > > > > cpu threads already need to have, so we're not making the possible
> > > > > > inadequacy of that information any worse than it already was.
> > > > > >
> > > > > > Examples.. so I'm not really sure how to write QOM objects, but I
> > > > > > hope
> > > > > > this is clear enough:
> > > > > >
> > > > > > On x86
> > > > > > .../cpu-package[0] (type 'acpi-thread')
> > > > > > present = true
> > > > > > location[0] = (node 0, socket 0, core 0, thread 0)
> > > > > > thread[0] = <link to cpu thread object>
> > > > > > .../cpu-package[1] (type 'acpi-thread')
> > > > > > present = false
> > > > > > location[0] = (node 0, socket 0, core 0, thread 1)
> > > > > >
> > > > > > On Power
> > > > > > .../cpu-package[0] (type 'spapr-core')
> > > > > > present = true
> > > > > > location[0] = (node 0, socket 0, core 0, thread 0)
> > > > > > location[1] = (node 0, socket 0, core 0, thread 1)
> > > > > > ...
> > > > > > location[7] = (node 0, socket 0, core 0, thread 7)
> > > > > > thread[0] = <link...>
> > > > > > ...
> > > > > > thread[7] = >link...>
> > > > > > .../cpu-package[1] (type 'spapr-core')
> > > > > > present = false
> > > > > > location[0] = (node 0, socket 0, core 0, thread 0)
> > > > > > location[1] = (node 0, socket 0, core 0, thread 1)
> > > > > > ...
> > > > > > location[7] = (node 0, socket 0, core 0, thread 7)
> > > > > >
> > > > > > Does that make sense?
> > > > > >
> > > > > > > > > or
> > > > > > > > > * via QMP/HMP command that would provide the same information
> > > > > > > > > only without need to precreate anything. The only
> > > > > > > > > difference
> > > > > > > > > is that it allows to use -device/device_add for new CPUs.
> > > > > > > > >
> > > > > > > >
> > > > > > > > I'd be ok with that option as well. I'd be thinking it would be
> > > > > > > > implemented via a class method on the package object which
> > > > > > > > returns the
> > > > > > > > addresses that its contained threads will have, whether or not
> > > > > > > > they're
> > > > > > > > present right now. Does that make sense?
> > > > > > > In this RFC it's MachineClass.possible_cpus method which is a bit
> > > > > > > more
> > > > > > > flexible as it allows a board to describe possible CPU devices
> > > > > > > (whatever
> > > > > > > they might be: sockets|cores|threads|some_chip_module) and their
> > > > > > > properties
> > > > > > > without forcing board to precreate cpu_package objects which
> > > > > > > should convey
> > > > > > > the same info one way or another.
> > > > > >
> > > > > > Hmm.. so my RFC so far (at least the revised version based on
> > > > > > Eduardo's comments) is that the cpu_package objects are always
> > > > > > precreated. In future we might allow dynamic construction, but that
> > > > > > will require a bunch more thinking to designt the right interfaces.
> > > > > >
> > > > > > > > > Considering that we would need to create HMP command so user
> > > > > > > > > could
> > > > > > > > > inspect possible CPUs from monitor, it would need to do the
> > > > > > > > > same as
> > > > > > > > > QMP command regardless of whether it's cpu-package objects or
> > > > > > > > > just board calculated info a runtime.
> > > > > > > > >
> > > > > > > > > > In the design Eduardo and I have been discussing we're
> > > > > > > > > > actually not
> > > > > > > > > > planning to allow device_add to construct CPU packages - at
> > > > > > > > > > least, not
> > > > > > > > > > for the time being. The idea is that the machine type will
> > > > > > > > > > construct
> > > > > > > > > > enough packages for maxcpus, and management just toggles
> > > > > > > > > > them on and
> > > > > > > > > > off.
> > > > > > > > > Another question is how it would work wrt migration?
> > > > > > > >
> > > > > > > > I'm assuming the "present" bits would be added to the migration
> > > > > > > > stream; seems straightforward enough to me. Is there some
> > > > > > > > consideration I'm missing?
> > > > > > > It's hard to estimate how cpu-package objects might complicate
> > > > > > > migration. It should not break migration for old machine types
> > > > > > > and if possible it should work for backwards migration to older
> > > > > > > QEMU versions (to be downstream friendly).
> > > > > >
> > > > > > So, the simple way to achieve that is to only instantiate the
> > > > > > cpu-package objects on newer machine types. Older machine types
> > > > > > will
> > > > > > instatiate the cpu threads directly from the machine type in the old
> > > > > > way, and (except for x86) won't allow cpu hotplug.
> > > > > >
> > > > > > I think that's a reasonable first approach. Later we can look at
> > > > > > migrating a non-package setup to a package setup, if it looks like
> > > > > > that will be useful.
> > > > > >
> > > > > > > If we go typical '-device/device_add
> > > > > > > whatever_cpu_device,foo_options_list'
> > > > > > > route then it would allow us to replicate older device models
> > > > > > > without
> > > > > > > issues (I don't expect any in x86 case) as it's what CPUs are now
> > > > > > > under the hood.
> > > > > > > This RFC doesn't force us to re-factor device models in order to
> > > > > > > use
> > > > > > > hotplug (where CPU objects are already self-sufficient
> > > > > > > devices/hotplug capable).
> > > > > > >
> > > > > > > It rather tries completely split interface aspect from how we are
> > > > > > > internally model CPU hotplug, and tries to solve issue with
> > > > > > >
> > > > > > > -device/device_add for which we need to provide
> > > > > > > 'what type to plug' and 'where to plug, which options to set
> > > > > > > to what'
> > > > > > >
> > > > > > > It's 1st level per you proposal, later we can do 2nd level on top
> > > > > > > of it
> > > > > > > using cpu-packages(flip present property) to simplify mgmt's job
> > > > > > > if it still would really needed (i.e. mgmt won't be able to cope
> > > > > > > with
> > > > > > > -device, which it already has support for).
> > > > > >
> > > > > > Yeah.. so the thing is, in the short term I'm really more interested
> > > > > > in the 2nd layer interface. It's something we can actually use,
> > > > > > whereas the 1st layer interfaace still has a lot of potential
> > > > > > complications.
> > > > > What complications do you see from POWER point if view?
> > > >
> > > > I don't relaly see any complications specific to Power. But the
> > > > biggest issue, as far as I can tell is how do we advertise to the user
> > > > / management layer what sorts of CPUs can be hotplugged - how many,
> > > > what types are possible and so forth. The constraints here could in
> > > > theory be pretty complex.
> > > that's what query-hotpluggable-cpus does, but not for theoretical
> > > set of platforms but rather a practical set that we a wanting
> > > CPU hotplug for.
> > > i.e. board returns a fixed board layout describing what cpu types
> > > could be hotplugged and where at in terms of [socket/core/thread]
> > > tuples, which maps well to current targets which need CPU hotplug
> > > (power/s390/x86/ARM).
> > >
> > > The rest of interface (i.e.) add/remove actions are handled by
> > > reused -device/device_add - that mgmt has already support for and
> > > works pretty well for migration as well
> > > (no need to maintain machine version-ed compat glue is plus).
> > >
> > > So any suggestions how to improve layout description returned
> > > by query-hotpluggable-cpus command are welcome.
> > > Even if we end up using QOM interface, suggestions will still
> > > be useful as the other interface will need to convey the same info
> > > just via other means.
> >
> > Yeah, as I mentioned elsewhere, I'm starting to come around to this
> > basic approach, although I'm still a bit dubious about the specific
> > format suggested. I don't have specific suggestions to improve it
> > yet, but I'm working on it :).
> >
> >
> > > > > > This is why Eduardo suggested - and I agreed - that it's probably
> > > > > > better to implement the "1st layer" as an internal
> > > > > > structure/interface
> > > > > > only, and implement the 2nd layer on top of that. When/if we need
> > > > > > to
> > > > > > we can revisit a user-accessible interface to the 1st layer.
> > > > > We are going around QOM based CPU introspecting interface for
> > > > > years now and that's exactly what 2nd layer is, just another
> > > > > implementation. I've just lost hope in this approach.
> > > > >
> > > > > What I'm suggesting in this RFC is to forget controversial
> > > > > QOM approach for now and use -device/device_add + QMP introspection,
> > > > > i.e. completely split interface from how boards internally implement
> > > > > CPU hotplug.
> > > >
> > > > I can see the appeal of that approach at this juncture. Hmm..
> > > A lot of work has been done to make CPUs device_add compatible.
> >
> > So... it's been much discussed, but I'm still pretty unclear on how
> > the device_add interface is supposed to work; at least in the context
> > of non thread-granularity hotplug.
> >
> > Basically, is it acceptable for:
> > device_add vendor-model-cpu-core
> >
> > to create, in addition to the core device, a bunch of additional
> > devices (the individual threads), or is that the "object mutating its
> > own topology" that Andreas objects to violently?
> I think it's acceptable to have vendor-model-cpu-core device
> considering it's platform limitation or socket if device model calls for it.
> I'm not sure that mutating applies to all objects but for Device
> inherited classes there shouldn't be any.
> i.e.
> 1. create Device with instance_init - constructor that shouldn't fail ever
> 2. set properties -
> done by -device/device_add and also by device_post_init() for globals
> 3. set 'realize' property to ON - allowed to fail, completes device
> initialization
> realize() hook must validate set earlier properties if it hasn't been
> done earlier and complete all child objects initialization,
Ok, does that include the initial construction of child objects?
> children are should be at 'realized' state when parent's realize()
> hook finishes without error. No further children are allowed to be
> created and not properties are allowed to be set after Device is realized.
> 4. Once realize() hook is executed, Device core code calls
> plug hook if it supported hotplug_handler_plug() which usually
> does the job of wiring Device to board. For more details see
> device_set_realized().
>
> On top of that Andreas would like that children weren't dynamically
> allocated but embedded into parent, included in parent's
> instance_size if possible i.e. children count is known at
> instance_init() time.
Right, which is not possible if we have a nr_threads property, as we
want for the cases we're looking at now.
> > If that is acceptable, where exactly should it be done? In the
> > device's instance_init? in realize? somewhere else?
> Not sure what question is about, does above answer it?
>
> > > The missing piece is letting mgmt to know what CPUs and with
> > > which options could be plugged in.
> >
> > Well, that's *a* missing piece, certainly..
> >
> > > And adding a query-hotpluggable-cpus QMP command looks like
> > > a path of the least resistance that would work for power/s390/x86/ARM.
> > >
> >
>
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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- Re: [Qemu-devel] [RFC] QMP: add query-hotpluggable-cpus, (continued)
- Re: [Qemu-devel] [RFC] QMP: add query-hotpluggable-cpus, Igor Mammedov, 2016/02/23
- Re: [Qemu-devel] [RFC] QMP: add query-hotpluggable-cpus, Eduardo Habkost, 2016/02/23
- Re: [Qemu-devel] [RFC] QMP: add query-hotpluggable-cpus, David Gibson, 2016/02/23
- Re: [Qemu-devel] [RFC] QMP: add query-hotpluggable-cpus, Igor Mammedov, 2016/02/24
- Re: [Qemu-devel] [RFC] QMP: add query-hotpluggable-cpus, David Gibson, 2016/02/24
- Re: [Qemu-devel] [RFC] QMP: add query-hotpluggable-cpus, Eduardo Habkost, 2016/02/25
- Re: [Qemu-devel] [RFC] QMP: add query-hotpluggable-cpus, David Gibson, 2016/02/23
- Re: [Qemu-devel] [RFC] QMP: add query-hotpluggable-cpus, Igor Mammedov, 2016/02/24
- Re: [Qemu-devel] [RFC] QMP: add query-hotpluggable-cpus, David Gibson, 2016/02/24
- Re: [Qemu-devel] [RFC] QMP: add query-hotpluggable-cpus, Igor Mammedov, 2016/02/25
- Re: [Qemu-devel] [RFC] QMP: add query-hotpluggable-cpus,
David Gibson <=
- Re: [Qemu-devel] [RFC] QMP: add query-hotpluggable-cpus, Igor Mammedov, 2016/02/26
- Re: [Qemu-devel] [RFC] QMP: add query-hotpluggable-cpus, Markus Armbruster, 2016/02/24
- Re: [Qemu-devel] [RFC] QMP: add query-hotpluggable-cpus, Igor Mammedov, 2016/02/24