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

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Message-ID: <a1276261-353a-f440-bc13-66fd49e08681@linux.ibm.com>
Date:   Wed, 5 Jul 2023 13:53:10 +0200
From:   Laurent Dufour <ldufour@...ux.ibm.com>
To:     "Zhang, Rui" <rui.zhang@...el.com>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
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

Le 05/07/2023 à 05:04, Zhang, Rui a écrit :
> 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.

Hi Rui,

Thanks for your interest for this series.
The latest Thomas's changes came into the PowerPC next branch.
I'm working on a rebase.

Cheers,
Laurent.

> 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
>>
>