linux-kernel - Re: [PATCH v3 0/9] Introduce SMT level and add PowerPC support

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Message-ID: <44bf8b00c960473e37e35571104b5a1b24673142.camel@intel.com>
Date:   Wed, 5 Jul 2023 03:04:34 +0000
From:   "Zhang, Rui" <rui.zhang@...el.com>
To:     "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "ldufour@...ux.ibm.com" <ldufour@...ux.ibm.com>
CC:     "npiggin@...il.com" <npiggin@...il.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "christophe.leroy@...roup.eu" <christophe.leroy@...roup.eu>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: [PATCH v3 0/9] Introduce SMT level and add PowerPC support

Hi, Laurent,

I want to test this patch set and found that it does not apply on top
of latest usptream git, because of some changes in this merge window,
so better rebase.

thanks,
rui

On Thu, 2023-06-29 at 16:31 +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.
> 
> 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 (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
>