hw/core/machine-smp.c | 48 +- hw/core/machine.c | 22 + hw/s390x/cpu-topology.c | 754 +++++++++++++++++++++++++++++ hw/s390x/meson.build | 1 + hw/s390x/s390-virtio-ccw.c | 77 ++- include/hw/boards.h | 8 + include/hw/s390x/cpu-topology.h | 99 ++++ include/hw/s390x/s390-virtio-ccw.h | 6 + include/hw/s390x/sclp.h | 1 + linux-headers/asm-s390/kvm.h | 9 + linux-headers/linux/kvm.h | 1 + qapi/machine.json | 14 +- qemu-options.hx | 6 +- softmmu/vl.c | 6 + target/s390x/cpu-sysemu.c | 7 + target/s390x/cpu.h | 52 ++ target/s390x/cpu_models.c | 1 + target/s390x/cpu_topology.c | 169 +++++++ target/s390x/kvm/kvm.c | 93 ++++ target/s390x/kvm/kvm_s390x.h | 2 + target/s390x/meson.build | 1 + 21 files changed, 1359 insertions(+), 18 deletions(-) create mode 100644 hw/s390x/cpu-topology.c create mode 100644 include/hw/s390x/cpu-topology.h create mode 100644 target/s390x/cpu_topology.c
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.
The design
==========
1) To be ready for hotplug, I chose an Object oriented design
of the topology containers:
- A node is a bridge on the SYSBUS and defines a "node bus"
- A drawer is hotplug on the "node bus"
- A book on the "drawer bus"
- A socket on the "book bus"
- And the CPU Topology List Entry (CPU-TLE)sits on the socket bus.
These objects will be enhanced with the cache information when
NUMA is implemented.
This also allows for easy retrieval when building the different SYSIB
for Store Topology System Information (STSI)
2) Perform Topology Function (PTF) instruction is made available to the
guest with a new KVM capability and intercepted in QEMU, allowing the
guest to pool for topology changes.
Features
========
- There is no direct match between IDs shown by:
- lscpu (unrelated numbered list),
- SYSIB 15.1.x (topology ID)
- The CPU number, left column of lscpu, is used to reference a CPU
by Linux tools
While the CPU address is used by QEMU for hotplug.
- Effect of -smp parsing on the topology with an example:
-smp 9,sockets=4,cores=4,maxcpus=16
We have 4 socket each holding 4 cores so that we have a maximum
of 16 CPU, 9 of them are active on boot. (Should be obvious)
# lscpu -e
CPU NODE DRAWER BOOK SOCKET CORE L1d:L1i:L2d:L2i ONLINE CONFIGURED POLARIZATION ADDRESS
0 0 0 0 0 0 0:0:0:0 yes yes horizontal 0
1 0 0 0 0 1 1:1:1:1 yes yes horizontal 1
2 0 0 0 0 2 2:2:2:2 yes yes horizontal 2
3 0 0 0 0 3 3:3:3:3 yes yes horizontal 3
4 0 0 0 1 4 4:4:4:4 yes yes horizontal 4
5 0 0 0 1 5 5:5:5:5 yes yes horizontal 5
6 0 0 0 1 6 6:6:6:6 yes yes horizontal 6
7 0 0 0 1 7 7:7:7:7 yes yes horizontal 7
8 0 0 0 2 8 8:8:8:8 yes yes horizontal 8
#
- To plug a new CPU inside the topology one can simply use the CPU
address like in:
(qemu) device_add host-s390x-cpu,core-id=12
# lscpu -e
CPU NODE DRAWER BOOK SOCKET CORE L1d:L1i:L2d:L2i ONLINE CONFIGURED POLARIZATION ADDRESS
0 0 0 0 0 0 0:0:0:0 yes yes horizontal 0
1 0 0 0 0 1 1:1:1:1 yes yes horizontal 1
2 0 0 0 0 2 2:2:2:2 yes yes horizontal 2
3 0 0 0 0 3 3:3:3:3 yes yes horizontal 3
4 0 0 0 1 4 4:4:4:4 yes yes horizontal 4
5 0 0 0 1 5 5:5:5:5 yes yes horizontal 5
6 0 0 0 1 6 6:6:6:6 yes yes horizontal 6
7 0 0 0 1 7 7:7:7:7 yes yes horizontal 7
8 0 0 0 2 8 8:8:8:8 yes yes horizontal 8
9 - - - - - ::: no yes horizontal 12
# chcpu -e 9
CPU 9 enabled
# lscpu -e
CPU NODE DRAWER BOOK SOCKET CORE L1d:L1i:L2d:L2i ONLINE CONFIGURED POLARIZATION ADDRESS
0 0 0 0 0 0 0:0:0:0 yes yes horizontal 0
1 0 0 0 0 1 1:1:1:1 yes yes horizontal 1
2 0 0 0 0 2 2:2:2:2 yes yes horizontal 2
3 0 0 0 0 3 3:3:3:3 yes yes horizontal 3
4 0 0 0 1 4 4:4:4:4 yes yes horizontal 4
5 0 0 0 1 5 5:5:5:5 yes yes horizontal 5
6 0 0 0 1 6 6:6:6:6 yes yes horizontal 6
7 0 0 0 1 7 7:7:7:7 yes yes horizontal 7
8 0 0 0 2 8 8:8:8:8 yes yes horizontal 8
9 0 0 0 3 9 9:9:9:9 yes yes horizontal 12
#
It is up to the admin level, Libvirt for example, to pin the righ CPU to the right
vCPU, but as we can see without NUMA, chosing separate sockets for CPUs is not easy
without hotplug because without information the code will assign the vCPU and fill
the sockets one after the other.
Note that this is also the default behavior on the LPAR.
Conclusion
==========
This patch, together with the associated KVM patch allows to provide CPU topology
information to the guest.
Currently, only dedicated vCPU and CPU are supported and a NUMA topology can only
be handled using CPU hotplug inside the guest.
Regards,
Pierre
Pierre Morel (12):
Update Linux Headers
s390x/cpu_topology: CPU topology objects and structures
s390x/cpu_topology: implementating Store Topology System Information
s390x/cpu_topology: Adding books to CPU topology
s390x/cpu_topology: Adding books to STSI
s390x/cpu_topology: Adding drawers to CPU topology
s390x/cpu_topology: Adding drawers to STSI
s390x/cpu_topology: implementing numa for the s390x topology
target/s390x: interception of PTF instruction
s390x/cpu_topology: resetting the Topology-Change-Report
s390x/cpu_topology: CPU topology migration
s390x/cpu_topology: activating CPU topology
hw/core/machine-smp.c | 48 +-
hw/core/machine.c | 22 +
hw/s390x/cpu-topology.c | 754 +++++++++++++++++++++++++++++
hw/s390x/meson.build | 1 +
hw/s390x/s390-virtio-ccw.c | 77 ++-
include/hw/boards.h | 8 +
include/hw/s390x/cpu-topology.h | 99 ++++
include/hw/s390x/s390-virtio-ccw.h | 6 +
include/hw/s390x/sclp.h | 1 +
linux-headers/asm-s390/kvm.h | 9 +
linux-headers/linux/kvm.h | 1 +
qapi/machine.json | 14 +-
qemu-options.hx | 6 +-
softmmu/vl.c | 6 +
target/s390x/cpu-sysemu.c | 7 +
target/s390x/cpu.h | 52 ++
target/s390x/cpu_models.c | 1 +
target/s390x/cpu_topology.c | 169 +++++++
target/s390x/kvm/kvm.c | 93 ++++
target/s390x/kvm/kvm_s390x.h | 2 +
target/s390x/meson.build | 1 +
21 files changed, 1359 insertions(+), 18 deletions(-)
create mode 100644 hw/s390x/cpu-topology.c
create mode 100644 include/hw/s390x/cpu-topology.h
create mode 100644 target/s390x/cpu_topology.c
--
2.31.1
Changelog:
- since v7
- Coherence with the Linux patch series changes for MTCR get
(Pierre)
- check return values during new CPU creation
(Thomas)
- Improving codding style and argument usages
(Thomas)
- since v6
- Changes on smp args in qemu-options
(Daniel)
- changed comments in machine.jason
(Daniel)
- Added reset
(Janosch)
- since v5
- rebasing on newer QEMU version
- reworked most lines above 80 characters.
- since v4
- Added drawer and books to topology
- Added numa topology
- Added documentation
- since v3
- Added migration
(Thomas)
- Separated STSI instruction from KVM to prepare TCG
(Thomas)
- Take care of endianess to prepare TCG
(Thomas)
- Added comments on STSI CPU container and PFT instruction
(Thomas)
- Moved enabling the instructions as the last patch
(Thomas)
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? 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.
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
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? The socket-id is computed from the core-id, so it's fixed, is it not? > 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. 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. > > So yes now we only have a topology report on vCPU plug. > > > > > > > >> >
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. > 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 > >> 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. > 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. -- Pierre Morel IBM Lab Boeblingen
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. > >> 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? And so it's socket id doesn't either. > >> >>> 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. > >> 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. > >
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
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