[Libguestfs] [p2v PATCH 2/6] restrict vCPU topology to (a) fully populated physical, or (b) 1 * N * 1

[Laszlo Ersek]

On 09/08/22 15:36, Richard W.M. Jones wrote:
> (Adding Dan for input)
> 
> On Thu, Sep 08, 2022 at 03:23:41PM +0200, Laszlo Ersek wrote:
>> On 09/08/22 10:03, Richard W.M. Jones wrote:
>>> On Mon, Sep 05, 2022 at 01:25:27PM +0200, Laszlo Ersek wrote:
>>>> +  "p2v.vcpu.dense_topo" => manual_entry->new(
>>>> +    shortopt => "", # ignored for booleans
>>>> +    description => "
>>>> +Copy the physical machine's CPU topology, densely populated, to the
>>>> +guest.  Disabled by default.  If disabled, the C<p2v.vcpu.cores> setting
>>>> +takes effect.",
>>>
>>>
>>> I just realised I'm not completely sure what "densely populated"
>>> means here.  I think we should have a bit more explanation.
>>>
>>> How about something like:
>>>
>>>   "p2v.vcpu.dense_topo" => manual_entry->new(
>>>     shortopt => "", # ignored for booleans
>>>     description => "
>>> Copy the physical machine's complete CPU topology (sockets, cores and
>>> threads) to the guest.  Disabled by default.  If disabled, the
>>> C<p2v.vcpu.cores> setting takes effect.",
>>>
>>> (Which might also imply that we rename this something like
>>> "complete_topo" or "full_topo" but I'll leave that to you.)
>>
>> By "dense", I meant to express that there are no gaps in the onlining of
>> the CPU topology.
>>
>> Assume we have 2 sockets, 2 cores/socket, 2 theads/core. Assume CPU#1
>> (in socket#1) is hot-pluggable, but isn't currently plugged, only CPU#0
>> (in socket#0) is present -- making for 1*2*2 = 4 logical processors in
>> total. A physical machine may well boot like this. Then our topology is
>> 2*2*2, but we only have 4 logical processors, so the topology is not
>> densely populated. The language is supposed to express that in any such
>> case, we'll ignore the online / plugged / etc count, and we'll just grab
>> the static topology, and fully / densely populate it with logical
>> processors.
>>
>> "Complete topology" does not express this. Sticking with the above
>> example, the topology is already complete on the physical machine (we
>> have full information about the levels of the hierarchy), but it's not
>> densely populated.
>>
>> Another example would be 1 * 4 * 2 physical (a normal low-end machine by
>> today's standards), where the sysadmin disables (say) cores #1 and #2
>> using /sys/devices/system/cpu/cpu{1,2}/online. (I think this may even be
>> possible on the kernel command line, for whatever reason necessary.) In
>> this case, during conversion, if "dense_topo" is set, we carry over not
>> just the topology (= the 1 * 4 * 2 hierarchy), but we also densely
>> populate it (producing 8 logical processors in the conversion output,
>> disregarding the "gaps" on the source; i.e. that only 4 logical
>> processors were available on the physical machine originally.)
>>
>> I considered "complete", and thought it didn't express my intent. "Full"
>> is so-so -- my problem is it seems to have two meanings; one is in fact
>> what I'm trying to say with "dense", but the other meaning is just
>> "complete", which I don't find good.
>>
>> The choices p2v should offer are:
>>
>> - Just carry over a flat VCPU count N --> this maps to a 1 socket * N
>> cores/socket * 1 thread / core topology, fully populated.
>>
>> - Otherwise (i.e., when the dense_topo knob is enabled), convert the
>> original topology (S sockets * C/S cores/socket * T threads/core), *AND*
>> fully populate that topology (disregarding the original "online count"
>> on the physical machine, which may easily be less than the (S * C * T)
>> product.)
> 
> I think the "mot juste" has to express that we're trying to model as
> closely as possible the real physical topology.  (The denseness
> doesn't seem to be so important - are there many machines where CPUs
> are not online?

Well it's a possibility.

> Can that even happen when virt-p2v is running?)

I think so, yes; although it should be really rare (physically removed
CPU from multi-socket systems, or some cores offlined on the kernel
cmdline perhaps -- I think there could be reasons for that).

> 
> How about:
> 
> authentic_topo
> physical_topo
> accurate_topo
> 
> ...?
> 
> The patch is totally fine, we're just quibbling about the
> word "dense" :-)

I'll rename "dense_topo" to "phys_topo", and also adopt your suggestion
for the manual.

Thanks!
Laszlo