Subject: [PATCH 00/10] Introduce SMT level and add PowerPC support

From: Laurent Dufour <[email-masked]>
Date: Thu, 15 Jun 2023 17:46:25 +0200

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.

[1] https://lore.kernel.org/linuxppc-dev/[email-masked]/
[2] https://lore.kernel.org/linuxppc-dev/[email-masked]/

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

Michael Ellerman (9):
  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_threads_supported()
  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 +
 arch/powerpc/Kconfig                          |  1 +
 arch/powerpc/include/asm/topology.h           | 20 +++++
 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               |  8 +-
 arch/x86/kernel/cpu/bugs.c                    |  3 +-
 arch/x86/kernel/smpboot.c                     | 25 +++++-
 include/linux/cpu.h                           | 25 +-----
 include/linux/cpu_smt.h                       | 33 ++++++++
 kernel/cpu.c                                  | 83 +++++++++++++++----
 13 files changed, 187 insertions(+), 54 deletions(-)
 create mode 100644 include/linux/cpu_smt.h

-- 
2.41.0