linux-kernel - Re: [PATCH 0/2 RESEND] sched/topology: Optimize topology_span

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Date: Wed, 10 Apr 2024 06:27:04 -0700
From: Yury Norov <yury.norov@...il.com>
To: Kyle Meyer <kyle.meyer@....com>
Cc: linux-kernel@...r.kernel.org, andriy.shevchenko@...ux.intel.com,
	linux@...musvillemoes.dk, mingo@...hat.com, peterz@...radead.org,
	juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
	russ.anderson@....com, dimitri.sivanich@....com, steve.wahl@....com
Subject: Re: [PATCH 0/2 RESEND] sched/topology: Optimize topology_span_sane()

On Tue, Apr 09, 2024 at 10:52:48AM -0500, Kyle Meyer wrote:
> 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 (per non-NUMA scheduling domain level), decreasing the
> boot time by approximately 20 seconds and preventing the soft lockup on
> the mentioned systems.
> 
> RESEND because Valentin Schneider reported that PATCH 2/2 wasn't
> delivered to all recipients.

Hmm... And this one doesn't have #1, at least for me. Can you please
resend again, so we all will have the series as a whole?

Also, can you rephrase this from patch #1 because it confuses people:

> + * for_each_cpu_from - iterate over every cpu present in @mask, starting at @cpu

Maybe: iterate over CPUs present in a @mask greater than a @cpu?
Or similar.

Thanks,
Yury

> 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