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/20230524155630.794584-1-...@ellerman.id.au/ [2] https://lore.kernel.org/linuxppc-dev/20230331153905.31698-1-lduf...@linux.ibm.com/ 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
