Re: [PATCH v4 00/10] Introduce SMT level and add PowerPC support

["Zhang, Rui" <rui.zhang@intel.com>]

Hi, Laurent,

I ran into a boot hang regression with latest upstream code, and it
took me a while to bisect the offending commit and workaround it.

Now I have tested this patch series on an Intel RaptorLake Hybrid
platform (4 Pcores with HT and 4 Ecores without HT), and it works as
expected.

So, for patch 1~7 in this series,

Tested-by: Zhang Rui <rui.zhang@intel.com>

thanks,
rui

On Wed, 2023-07-05 at 16:51 +0200, Laurent Dufour wrote:
> 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.
> 
> v4:
>   Rebase on top of 6.5's updates
>   Remove a dependancy against the X86's symbol
> cpu_primary_thread_mask
> 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@ellerman.id.au/
> [2]
> https://lore.kernel.org/linuxppc-dev/20230331153905.31698-1-ldufour@linux.ibm.com/
> 
> 
> Laurent Dufour (2):
>   cpu/hotplug: remove dependancy against cpu_primary_thread_mask
>   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/common.c                  |   2 +-
>  arch/x86/kernel/smpboot.c                     |   8 -
>  include/linux/cpu.h                           |  25 +--
>  include/linux/cpu_smt.h                       |  33 ++++
>  kernel/cpu.c                                  | 142 +++++++++++++---
> --
>  15 files changed, 196 insertions(+), 85 deletions(-)
>  create mode 100644 include/linux/cpu_smt.h
>