On 7/15/22 20:28, Janis Schoetterl-Glausch wrote:
On 7/15/22 15:47, Pierre Morel wrote:
On 7/15/22 11:31, Janis Schoetterl-Glausch wrote:
On 7/14/22 22:05, Pierre Morel wrote:
On 7/14/22 20:43, Janis Schoetterl-Glausch wrote:
On 6/20/22 16:03, Pierre Morel wrote:
Hi,
This new spin is essentially for coherence with the last Linux CPU
Topology patch, function testing and coding style modifications.
Forword
=======
The goal of this series is to implement CPU topology for S390, it
improves the preceeding series with the implementation of books and
drawers, of non uniform CPU topology and with documentation.
To use these patches, you will need the Linux series version 10.
You find it there:
https://lkml.org/lkml/2022/6/20/590
Currently this code is for KVM only, I have no idea if it is interesting
to provide a TCG patch. If ever it will be done in another series.
To have a better understanding of the S390x CPU Topology and its
implementation in QEMU you can have a look at the documentation in the
last patch or follow the introduction here under.
A short introduction
====================
CPU Topology is described in the S390 POP with essentially the description
of two instructions:
PTF Perform Topology function used to poll for topology change
and used to set the polarization but this part is not part of this item.
STSI Store System Information and the SYSIB 15.1.x providing the Topology
configuration.
S390 Topology is a 6 levels hierarchical topology with up to 5 level
of containers. The last topology level, specifying the CPU cores.
This patch series only uses the two lower levels sockets and cores.
To get the information on the topology, S390 provides the STSI
instruction, which stores a structures providing the list of the
containers used in the Machine topology: the SYSIB.
A selector within the STSI instruction allow to chose how many topology
levels will be provide in the SYSIB.
Using the Topology List Entries (TLE) provided inside the SYSIB we
the Linux kernel is able to compute the information about the cache
distance between two cores and can use this information to take
scheduling decisions.
Do the socket, book, ... metaphors and looking at STSI from the existing
smp infrastructure even make sense?
Sorry, I do not understand.
I admit the cover-letter is old and I did not rewrite it really good since the first patch series.
What we do is:
Compute the STSI from the SMP + numa + device QEMU parameters .
STSI 15.1.x reports the topology to the guest and for a virtual machine,
this topology can be very dynamic. So a CPU can move from from one topology
container to another, but the socket of a cpu changing while it's running seems
a bit strange. And this isn't supported by this patch series as far as I understand,
the only topology changes are on hotplug.
A CPU changing from a socket to another socket is the only case the PTF instruction reports a change in the topology with the case a new CPU is plug in.
Can a CPU actually change between sockets right now?
To be exact, what I understand is that a shared CPU can be scheduled to another real CPU exactly as a guest vCPU can be scheduled by the host to another host CPU.
Ah, ok, this is what I'm forgetting, and what made communication harder,
there are two ways by which the topology can change:
1. the host topology changes
2. the vCPU threads are scheduled on another host CPU
I've been only thinking about the 2.
I assumed some outside entity (libvirt?) pins vCPU threads, and so it would
be the responsibility of that entity to set the topology which then is
reported to the guest. So if you pin vCPUs for the whole lifetime of the vm
then you could do that by specifying the topology up front with -devices.
If you want to support migration, then the outside entity would need a way
to tell qemu the updated topology.
Yes
The socket-id is computed from the core-id, so it's fixed, is it not?
the virtual socket-id is computed from the virtual core-id
Meaning cpu.env.core_id, correct? (which is the same as cpu.cpu_index which is the same as
ms->possible_cpus->cpus[core_id].props.core_id)
And a cpu's core id doesn't change during the lifetime of the vm, right?
right
And so it's socket id doesn't either.
Yes
It is not expected to appear often but it does appear.
The code has been removed from the kernel in spin 10 for 2 reasons:
1) we decided to first support only dedicated and pinned CPU> 2) Christian fears it may happen too often due to Linux host scheduling and could be a performance problem
This seems sensible, but now it seems too static.
For example after migration, you cannot tell the guest which CPUs are in the same socket, book, ...,
unless I'm misunderstanding something.
No, to do this we would need to ask the kernel about it.
You mean polling /sys/devices/system/cpu/cpu*/topology/*_id ?
That should work if it isn't done to frequently, right?
And if it's done by the entity doing the pinning it could judge if the host topology change
is relevant to the guest and if so tell qemu how to update it.
yes, I guess we will need to change the core-id which may be complicated
or find another way to link the vCPU topology with the host CPU topology.
First I wanted to have something directly from the kernel, as we have
there all the info on vCPU and host topology.
That is why I had a struct in the UAPI.
Viktor is as you say here for something in userland only.
For the moment I would like to stay for this patch series on a fixed
topology, set by the admin, then we go on updating the topology.
And migration is rare, but something you'd want to be able to react to.
And I could imaging that the vCPUs are pinned most of the time, but the pinning changes occasionally.
I think on migration we should just make a kvm_set_mtcr on post_load like Nico suggested everything else seems complicated for a questionable benefit.
But what is the point? The result of STSI reported to the guest doesn't actually change, does it?
Since the same CPUs with the same calculated socket-ids, ..., exist.
You cannot migrate to a vm with a different virtual topology, since the CPUs get matched via the cpu_index
as far as I can tell, which is the same as the core_id, or am I misunderstanding something?
Migrating the MTCR bit is correct, if it is 1 than there was a cpu hotplug that the guest did not yet observe,
but setting it to 1 after migration would we wrong if the STSI result would be the same.
That is a good point, IIUC it follows that:
- a CPU hotplug can not be done during the migration.
- migration can not be started while a CPU is being hot plugged.
--
Pierre Morel
IBM Lab Boeblingen
