[PATCH 0/3] sched: Simplify ifdeffery around CONFIG_SCHED_SMT

[Shrikanth Hegde]

While working on paravirt series on maintaining preferred CPUs, I wanted
to add/remove a core. To do that, I had to access cpu_smt_mask. For that
had to use CONFIG_SCHED_SMT around the code. That made me think, for
CONFIG_SCHED_SMT=n, it effectively means to just add/remove a CPU. Thats when
I thought of this idea of using cpumask_of(cpu) when CONFIG_SCHED_SMT=n. 

Semantics
=========
- For CONFIG_SCHED_SMT=y:
    No functional change.
- For CONFIG_SCHED_SMT=n:
    - cpu_smt_mask(cpu) becomes cpumask_of(cpu), effectively making it
      per CPU with no siblings.
    - sched_smt_present remains defined, but never becomes active:
    	 Since cpumask_weight(cpumask_of(cpu)) == 1

Performance impact
==================
- CONFIG_SCHED_SMT=y:
    No change in generated code.
- CONFIG_SCHED_SMT=n:
    - Small increase in text size (~0.01%) due to removal of compile-time
      stubs. Most paths remain effectively dead due to static keys.
    - Fast paths are protected using IS_ENABLED(CONFIG_SCHED_SMT).

Testing
=======
- Did build/boot test on powerpc for CONFIG_SCHED_SMT=y/n.
- Ran hackbench on powerpc for CONFIG_SCHED_SMT=y/n. Didn't observe any
  major difference.
- Did build/boot test on x86 for CONFIG_SCHED_SMT=y/n. For x86 i had
  change the code for CONFIG_SCHED_SMT=n


Plus, Major distros make CONFIG_SCHED_SMT=y for all major
archs and few archs unconditionally make CONFIG_SCHED_SMT=y (x86,
s390), So CONFIG_SCHED_SMT=n is a rare case.

With that, cpu_smt_mask() to be used unconditionally and reduces
CONFIG_SCHED_SMT-specific code paths, improving readability and
maintainability.

Please review and consider if this simplification is a worthwhile cleanup.

Shrikanth Hegde (3):
  topology: Introduce cpu_smt_mask for CONFIG_SCHED_SMT=n
  sched: Simplify ifdeffery around cpu_smt_mask
  sched/fair: Add compile time check in fastpaths for CONFIG_SCHED_SMT=n

 include/linux/sched/smt.h |  4 ----
 include/linux/topology.h  | 15 +++++++++++++-
 kernel/sched/core.c       |  6 ------
 kernel/sched/ext_idle.c   |  6 ------
 kernel/sched/fair.c       | 41 +++++----------------------------------
 kernel/sched/sched.h      |  6 ------
 kernel/sched/topology.c   |  2 --
 kernel/stop_machine.c     |  2 --
 kernel/workqueue.c        |  4 ----
 9 files changed, 19 insertions(+), 67 deletions(-)

-- 
2.51.0