[PATCH v3 0/3] sched/topology: optimize topology_span_sane()

[Yury Norov <yury.norov@gmail.com>]

The function may call cpumask_equal with tl->mask(cpu) == tl->mask(i),
even when cpu != i. In such case, cpumask_equal() would always return
true, and we can proceed to the next iteration immediately.

Valentin Schneider shares on it:

  PKG can potentially hit that condition, and so can any
  sched_domain_mask_f that relies on the node masks...

  I'm thinking ideally we should have checks in place to
  ensure all node_to_cpumask_map[] masks are disjoint,
  then we could entirely skip the levels that use these
  masks in topology_span_sane(), but there's unfortunately
  no nice way to flag them... Also there would be cases
  where there's no real difference between PKG and NODE
  other than NODE is still based on a per-cpu cpumask and
  PKG isn't, so I don't see a nicer way to go about this.

v1: https://lore.kernel.org/lkml/ZrJk00cmVaUIAr4G@yury-ThinkPad/T/
v2: https://lkml.org/lkml/2024/8/7/1299
v3:
 - add topology_cpumask_equal() helper in #3;
 - re-use 'cpu' as an iterator int the for_each_cpu() loop;
 - add proper versioning for all patches.

Yury Norov (3):
  sched/topology: pre-compute topology_span_sane() loop params
  sched/topology: optimize topology_span_sane()
  sched/topology: reorganize topology_span_sane() checking order

 kernel/sched/topology.c | 29 +++++++++++++++++++++++++----
 1 file changed, 25 insertions(+), 4 deletions(-)

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2.43.0