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

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Message-ID: <ZrzcozFadC5xQdUq@yury-ThinkPad>
Date: Wed, 14 Aug 2024 09:35:01 -0700
From: Yury Norov <yury.norov@...il.com>
To: linux-kernel@...r.kernel.org
Cc: 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: Re: [PATCH v2 0/2] sched/topology: optimize topology_span_sane()

Ping?

On Wed, Aug 07, 2024 at 12:05:19PM -0700, Yury Norov wrote:
> 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