linux-kernel - [PATCH v2 0/2] sched/topology: optimize topology_span

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Message-ID: <20240807190522.432388-1-yury.norov@gmail.com>
Date: Wed,  7 Aug 2024 12:05:19 -0700
From: Yury Norov <yury.norov@...il.com>
To: linux-kernel@...r.kernel.org
Cc: Yury Norov <yury.norov@...il.com>,
	Chen Yu <yu.c.chen@...el.com>,
	Christophe JAILLET <christophe.jaillet@...adoo.fr>,
	Leonardo Bras <leobras@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>,
	Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>
Subject: [PATCH v2 0/2] sched/topology: optimize topology_span_sane()

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:
 - defer initialization of 'mc' in patch #1 @Chen Yu;
 - more comments from Valentin Schneider.

Yury Norov (2):
  sched/topology: pre-compute topology_span_sane() loop params
  sched/topology: optimize topology_span_sane()

 kernel/sched/topology.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

-- 
2.43.0