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From: Kyle Meyer <kyle.meyer@hpe.com>
To: linux-kernel@vger.kernel.org, yury.norov@gmail.com,
        andriy.shevchenko@linux.intel.com, linux@rasmusvillemoes.dk,
        mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
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        bristot@redhat.com, vschneid@redhat.com
Cc: russ.anderson@hpe.com, dimitri.sivanich@hpe.com, steve.wahl@hpe.com,
        Kyle Meyer <kyle.meyer@hpe.com>
Subject: [PATCH v2 0/2] sched/topology: Optimize topology_span_sane()
Date: Wed, 10 Apr 2024 16:33:09 -0500
Message-Id: <20240410213311.511470-1-kyle.meyer@hpe.com>
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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@hpe.com/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(-)

-- 
2.44.0