[v12,0/4] Support SMT control on arm64

Message

Yicong Yang March 11, 2025, 7:51 a.m. UTC

From: Yicong Yang <[e-mail]>

The core CPU control framework supports runtime SMT control which
is not yet supported on arm64. Besides the general vulnerabilities
concerns we want this runtime control on our arm64 server for:

- better single CPU performance in some cases
- saving overall power consumption

This patchset implements it in the following aspects:

- Provides a default topology_is_primary_thread()
- support retrieve SMT thread number on OF based system
- support retrieve SMT thread number on ACPI based system
- select HOTPLUG_SMT for arm64

Tests has been done on our ACPI based arm64 server and on ACPI/OF
based QEMU VMs.

Change since v11:
- Remove the check and warning for heterogeneous platform as suggested and discussed
- refine comments/commit according to Dietmar
Link: https://lore.kernel.org/linux-arm-kernel/[e-mail]/

Change since v10:
- handle topology parsing failure case on DT based system
- address some style comments per Jonathan and add tags, Thanks
Link: https://lore.kernel.org/linux-arm-kernel/[e-mail]/

Change since v9:
- Refine the comment of topology_is_primary_thread(). Tested with LoongArch
  to prove it also works on architecture's not using CONFIG_GENERIC_ARCH_TOPOLOGY
- always call cpu_smt_set_num_threads() to make the smt/control shows correct
  status on non-SMT system
Link: https://lore.kernel.org/linux-arm-kernel/[e-mail]/

Change since v8:
- Fix WARN on ACPI based non-SMT platform noticed in v7, per Pierre.
Link: https://lore.kernel.org/all/[e-mail]/

Change since v7:
Address the comments from Thomas:
- Add a newline between the glue define and function of topology_is_primary_thread
- Explicitly mention the sibling mask won't be empty in the comment
Link: https://lore.kernel.org/lkml/[e-mail]/

Change since v6:
- Fix unused variable if !CONFIG_ARM64 || !CONFIG_RISV found by lkp-test
- Fix max_smt_thread_num updating in OF path pointed by Pierre
- Drop unused variable and refine the comments/commit per Pierre
Link: https://lore.kernel.org/linux-arm-kernel/[e-mail]/

Change since v5:
- Drop the dependency on CONFIG_SMP since it's always on arm64, per Pierre
- Avoid potential multiple calls of cpu_smt_set_num_threads() on asymmetric system, per Dietmar
- Detect heterogeneous SMT topology and issue a warning for partly support, per Pierre
- Thanks Dietmar for testing, didn't pickup the tag due to code changes. Thanks testing by Pierre
Link: https://lore.kernel.org/linux-arm-kernel/[e-mail]/

Change since v4:
- Provide a default topology_is_primary_thread() in the framework, Per Will
Link: https://lore.kernel.org/linux-arm-kernel/[e-mail]/

Change since v3:
- Fix some build and kconfig error reported by kernel test robot <[e-mail]>
Link: https://lore.kernel.org/linux-arm-kernel/[e-mail]/

Change since v2:
- Detect SMT thread number at topology build from ACPI/DT, avoid looping CPUs
- Split patches into ACPI/OF/arch_topology path and enable the kconfig for arm64
Link: https://lore.kernel.org/linux-arm-kernel/[e-mail]/

Yicong Yang (4):
  cpu/SMT: Provide a default topology_is_primary_thread()
  arch_topology: Support SMT control for OF based system
  arm64: topology: Support SMT control on ACPI based system
  arm64: Kconfig: Enable HOTPLUG_SMT

 arch/arm64/Kconfig                  |  1 +
 arch/arm64/kernel/topology.c        | 54 +++++++++++++++++++++++++++++
 arch/powerpc/include/asm/topology.h |  1 +
 arch/x86/include/asm/topology.h     |  2 +-
 drivers/base/arch_topology.c        | 18 ++++++++++
 include/linux/topology.h            | 24 +++++++++++++
 6 files changed, 99 insertions(+), 1 deletion(-)

Comments

Sudeep Holla March 11, 2025, 2:41 p.m. UTC | #1

On Tue, Mar 11, 2025 at 03:51:39PM +0800, Yicong Yang wrote:
> From: Yicong Yang <[e-mail]>
> 
> The core CPU control framework supports runtime SMT control which
> is not yet supported on arm64. Besides the general vulnerabilities
> concerns we want this runtime control on our arm64 server for:
> 
> - better single CPU performance in some cases
> - saving overall power consumption
> 
> This patchset implements it in the following aspects:
> 
> - Provides a default topology_is_primary_thread()
> - support retrieve SMT thread number on OF based system
> - support retrieve SMT thread number on ACPI based system
> - select HOTPLUG_SMT for arm64
> 
> Tests has been done on our ACPI based arm64 server and on ACPI/OF
> based QEMU VMs.
> 
> Change since v11:
> - Remove the check and warning for heterogeneous platform as suggested and discussed

IIUC, the ask was not to remove it completely but to allow single
threaded and same number of threads in all the multi-threaded CPUs.
Anyways that is not a must, we can just stash max_smt_thread_num value
before updating with max and see if it changes from one value to another
with value != 1. It can be done later when we want to warn such systems
if they appear in the future and we can't support them. For now, it looks
fine.