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

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Date:   Wed, 26 Apr 2023 10:17:12 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Yury Norov <yury.norov@...il.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 25/04/23 22:26, Yury Norov wrote:
> 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.

I'd hope so in 2023! I still prefer the iterative approach as I find it
more readable, but I'm not /too/ strongly attached to it.