linux-kernel - Re: [PATCH] sched/topology: improve topology_span

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Message-ID: <20241015110544.GQ16066@noisy.programming.kicks-ass.net>
Date: Tue, 15 Oct 2024 13:05:44 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: K Prateek Nayak <kprateek.nayak@....com>
Cc: Steve Wahl <steve.wahl@....com>, Russ Anderson <rja@....com>,
	Dimitri Sivanich <sivanich@....com>, Ingo Molnar <mingo@...hat.com>,
	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>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/topology: improve topology_span_sane speed

On Tue, Oct 15, 2024 at 03:50:49PM +0530, K Prateek Nayak wrote:

> > +	masks = kmalloc_array(num_possible_cpus(), sizeof(struct cpumask *), GFP_KERNEL);
> > +	if (!masks)
> > +		return ret;
> 
> That looks like a very large array that seems unnecessary. Instead, is
> it possible to use "tl->mask(id)" down blow to check for equality? (I'll
> elaborate more below)

drive-by comments, haven't got time atm to read this, but
num_possible_cpus() is wrong, this should be nr_cpu_ids.

The CPU mask must not be assumed dense.