[PATCH v2 0/2] Skip offline cores when enabling SMT on PowerPC

["Nysal Jan K.A." <nysal@linux.ibm.com>]

From: "Nysal Jan K.A" <nysal@linux.ibm.com>

After the addition of HOTPLUG_SMT support for PowerPC [1] there was a
regression reported [2] when enabling SMT. On a system with at least
one offline core, when enabling SMT, the expectation is that no CPUs
of offline cores are made online.

On a POWER9 system with 4 cores in SMT4 mode:
$ ppc64_cpu --info
Core   0:    0*    1*    2*    3*
Core   1:    4*    5*    6*    7*
Core   2:    8*    9*   10*   11*
Core   3:   12*   13*   14*   15*

Turn only one core on:
$ ppc64_cpu --cores-on=1
$ ppc64_cpu --info
Core   0:    0*    1*    2*    3*
Core   1:    4     5     6     7
Core   2:    8     9    10    11
Core   3:   12    13    14    15

Change the SMT level to 2:
$ ppc64_cpu --smt=2
$ ppc64_cpu --info
Core   0:    0*    1*    2     3
Core   1:    4     5     6     7
Core   2:    8     9    10    11
Core   3:   12    13    14    15

As expected we see only two CPUs of core 0 are online

Change the SMT level to 4:
$ ppc64_cpu --smt=4
$ ppc64_cpu --info
Core   0:    0*    1*    2*    3*
Core   1:    4*    5*    6*    7*
Core   2:    8*    9*   10*   11*
Core   3:   12*   13*   14*   15*

The CPUs of offline cores are made online. If a core is offline then
enabling SMT should not online CPUs of this core. An arch specific
function topology_is_core_online() is proposed to address this.

topology_sibling_cpumask() might not be available yet for the to be
brought up CPU, so it cannot be relied upon to detect an offline core.

v2:
 * Document PowerPC specific behaviour when enabling SMT

v1:
 https://lore.kernel.org/all/20240612185046.1826891-1-nysal@linux.ibm.com/

[1] https://lore.kernel.org/lkml/20230705145143.40545-1-ldufour@linux.ibm.com/
[2] https://groups.google.com/g/powerpc-utils-devel/c/wrwVzAAnRlI/m/5KJSoqP4BAAJ

Nysal Jan K.A (2):
  cpu/SMT: Enable SMT only if a core is online
  powerpc/topology: Check if a core is online

 Documentation/ABI/testing/sysfs-devices-system-cpu |  3 ++-
 arch/powerpc/include/asm/topology.h                | 13 +++++++++++++
 kernel/cpu.c                                       | 12 +++++++++++-
 3 files changed, 26 insertions(+), 2 deletions(-)


base-commit: 94ede2a3e9135764736221c080ac7c0ad993dc2d
-- 
2.35.3