netdev - Re: [PATCH v2 2/8] sched/topology: introduce sched_numa_find_next

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Date:   Tue, 25 Apr 2023 22:26:23 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Valentin Schneider <vschneid@...hat.com>
Cc:     Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
        Saeed Mahameed <saeedm@...dia.com>,
        Pawel Chmielewski <pawel.chmielewski@...el.com>,
        Leon Romanovsky <leon@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        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>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Tariq Toukan <tariqt@...dia.com>,
        Gal Pressman <gal@...dia.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Barry Song <baohua@...nel.org>
Subject: Re: [PATCH v2 2/8] sched/topology: introduce
 sched_numa_find_next_cpu()

On Tue, Apr 25, 2023 at 10:54:56AM +0100, Valentin Schneider wrote:
> On 19/04/23 22:19, Yury Norov wrote:
> > +/*
> > + * sched_numa_find_next_cpu() - given the NUMA topology, find the next cpu
> > + * cpumask: cpumask to find a cpu from
> > + * cpu: current cpu
> > + * node: local node
> > + * hop: (in/out) indicates distance order of current CPU to a local node
> > + *
> > + * The function searches for next cpu at a given NUMA distance, indicated
> > + * by hop, and if nothing found, tries to find CPUs at a greater distance,
> > + * starting from the beginning.
> > + *
> > + * Return: cpu, or >= nr_cpu_ids when nothing found.
> > + */
> > +int sched_numa_find_next_cpu(const struct cpumask *cpus, int cpu, int node, unsigned int *hop)
> > +{
> > +	unsigned long *cur, *prev;
> > +	struct cpumask ***masks;
> > +	unsigned int ret;
> > +
> > +	if (*hop >= sched_domains_numa_levels)
> > +		return nr_cpu_ids;
> > +
> > +	masks = rcu_dereference(sched_domains_numa_masks);
> > +	cur = cpumask_bits(masks[*hop][node]);
> > +	if (*hop == 0)
> > +		ret = find_next_and_bit(cpumask_bits(cpus), cur, nr_cpu_ids, cpu);
> > +	else {
> > +		prev = cpumask_bits(masks[*hop - 1][node]);
> > +		ret = find_next_and_andnot_bit(cpumask_bits(cpus), cur, prev, nr_cpu_ids, cpu);
> > +	}
> > +
> > +	if (ret < nr_cpu_ids)
> > +		return ret;
> > +
> > +	*hop += 1;
> > +	return sched_numa_find_next_cpu(cpus, 0, node, hop);
> 
> sched_domains_numa_levels is a fairly small number, so the recursion depth
> isn't something we really need to worry about - still, the iterative
> variant of this is fairly straightforward to get to:

This is a tail recursion. Compiler normally converts it into the loop just
as well. At least, my GCC does.