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

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Message-ID: <ff068ea8-c731-3763-066b-2503ffddd0e8@linux.ibm.com>
Date:   Mon, 10 Jul 2023 11:08:03 +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 v4 00/10] Introduce SMT level and add PowerPC support



Le 09/07/2023 à 17:25, Zhang, Rui a écrit :
> 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@...el.com>

Thanks Rui!

> 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
>>
>