[PATCH v2 0/2] sched/topology: Optimize topology_span_sane()

[Kyle Meyer]

A soft lockup is being detected in build_sched_domains() on 32 socket
Sapphire Rapids systems with 3840 processors.

topology_span_sane(), called by build_sched_domains(), checks that each
processor's non-NUMA scheduling domains are completely equal or
completely disjoint. If a non-NUMA scheduling domain partially overlaps
another, scheduling groups can break.

This series adds for_each_cpu_from() as a generic cpumask macro to
optimize topology_span_sane() by removing duplicate comparisons. The
total number of comparisons is reduced from N * (N - 1) to
N * (N - 1) / 2 on each non-NUMA scheduling domain level, decreasing
the boot time by approximately 20 seconds and preventing the soft lockup
on the mentioned systems.

Changes in v2:
  * 1/2: Change for_each_cpu()'s description.
  * 2/2: Add more information to the commit message.
  * https://lore.kernel.org/linux-kernel/20240409155250.3660517-1-kyle.meyer@xxxxxxx/T/

Kyle Meyer (2):
  cpumask: Add for_each_cpu_from()
  sched/topology: Optimize topology_span_sane()

 include/linux/cpumask.h | 10 ++++++++++
 kernel/sched/topology.c |  6 ++----
 2 files changed, 12 insertions(+), 4 deletions(-)

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2.44.0