Re: [PATCH v8 00/12] s390x: CPU Topology

[Pierre Morel]

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.
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

So yes now we only have a topology report on vCPU plug.







>

--
Pierre Morel
IBM Lab Boeblingen