[PATCH v3 0/9] Introduce SMT level and add PowerPC support

[Laurent Dufour]

I'm taking over the series Michael sent previously [1] which is smartly
reviewing the initial series I sent [2].  This series is addressing the
comments sent by Thomas and me on the Michael's one.

Here is a short introduction to the issue this series is addressing:

When a new CPU is added, the kernel is activating all its threads. This
leads to weird, but functional, result when adding CPU on a SMT 4 system
for instance.

Here the newly added CPU 1 has 8 threads while the other one has 4 threads
active (system has been booted with the 'smt-enabled=4' kernel option):

ltcden3-lp12:~ # ppc64_cpu --info
Core   0:    0*    1*    2*    3*    4     5     6     7
Core   1:    8*    9*   10*   11*   12*   13*   14*   15*

This mixed SMT level may confused end users and/or some applications.

There is no SMT level recorded in the kernel (common code), neither in user
space, as far as I know. Such a level is helpful when adding new CPU or
when optimizing the energy efficiency (when reactivating CPUs).

When SMP and HOTPLUG_SMT are defined, this series is adding a new SMT level
(cpu_smt_num_threads) and few callbacks allowing the architecture code to
fine control this value, setting a max and a "at boot" level, and
controling whether a thread should be onlined or not.

v3:
  Fix a build error in the patch 6/9
v2:
  As Thomas suggested,
    Reword some commit's description
    Remove topology_smt_supported()
    Remove topology_smt_threads_supported()
    Introduce CONFIG_SMT_NUM_THREADS_DYNAMIC
    Remove switch() in __store_smt_control()
  Update kernel-parameters.txt

[1] https://lore.kernel.org/linuxppc-dev/20230524155630.794584-1-mpe@xxxxxxxxxxxxxx/
[2] https://lore.kernel.org/linuxppc-dev/20230331153905.31698-1-ldufour@xxxxxxxxxxxxx/

Laurent Dufour (1):
  cpu/SMT: Remove topology_smt_supported()

Michael Ellerman (8):
  cpu/SMT: Move SMT prototypes into cpu_smt.h
  cpu/SMT: Move smt/control simple exit cases earlier
  cpu/SMT: Store the current/max number of threads
  cpu/SMT: Create topology_smt_thread_allowed()
  cpu/SMT: Allow enabling partial SMT states via sysfs
  powerpc/pseries: Initialise CPU hotplug callbacks earlier
  powerpc: Add HOTPLUG_SMT support
  powerpc/pseries: Honour current SMT state when DLPAR onlining CPUs

 .../ABI/testing/sysfs-devices-system-cpu      |   1 +
 .../admin-guide/kernel-parameters.txt         |   4 +-
 arch/Kconfig                                  |   3 +
 arch/powerpc/Kconfig                          |   2 +
 arch/powerpc/include/asm/topology.h           |  15 +++
 arch/powerpc/kernel/smp.c                     |   8 +-
 arch/powerpc/platforms/pseries/hotplug-cpu.c  |  30 +++--
 arch/powerpc/platforms/pseries/pseries.h      |   2 +
 arch/powerpc/platforms/pseries/setup.c        |   2 +
 arch/x86/include/asm/topology.h               |   4 +-
 arch/x86/kernel/cpu/bugs.c                    |   3 +-
 arch/x86/kernel/smpboot.c                     |   8 --
 include/linux/cpu.h                           |  25 +---
 include/linux/cpu_smt.h                       |  33 +++++
 kernel/cpu.c                                  | 118 ++++++++++++++----
 15 files changed, 187 insertions(+), 71 deletions(-)
 create mode 100644 include/linux/cpu_smt.h

-- 
2.41.0