linux-kernel - Re: [PATCH 3/4] sched: add sched_numa_find_nth

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Date:   Thu, 10 Nov 2022 20:11:35 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     linux-kernel@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Barry Song <baohua@...nel.org>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Gal Pressman <gal@...dia.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Ingo Molnar <mingo@...hat.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Mel Gorman <mgorman@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Saeed Mahameed <saeedm@...dia.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tariq Toukan <tariqt@...dia.com>,
        Tariq Toukan <ttoukan.linux@...il.com>,
        Tony Luck <tony.luck@...el.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     linux-crypto@...r.kernel.org, netdev@...r.kernel.org,
        linux-rdma@...r.kernel.org
Subject: Re: [PATCH 3/4] sched: add sched_numa_find_nth_cpu()

On Thu, Nov 10, 2022 at 08:00:26PM -0800, Yury Norov wrote:
> The function finds Nth set CPU in a given cpumask starting from a given
> node.
> 
> Leveraging the fact that each hop in sched_domains_numa_masks includes the
> same or greater number of CPUs than the previous one, we can use binary
> search on hops instead of linear walk, which makes the overall complexity
> of O(log n) in terms of number of cpumask_weight() calls.
> 
> Signed-off-by: Yury Norov <yury.norov@...il.com>
> ---
>  include/linux/topology.h |  8 ++++++++
>  kernel/sched/topology.c  | 42 ++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 50 insertions(+)
> 
> diff --git a/include/linux/topology.h b/include/linux/topology.h
> index 4564faafd0e1..63048ac3207c 100644
> --- a/include/linux/topology.h
> +++ b/include/linux/topology.h
> @@ -245,5 +245,13 @@ static inline const struct cpumask *cpu_cpu_mask(int cpu)
>  	return cpumask_of_node(cpu_to_node(cpu));
>  }
>  
> +#ifdef CONFIG_NUMA
> +int sched_numa_find_nth_cpu(const struct cpumask *cpus, int cpu, int node);
> +#else
> +int sched_numa_find_nth_cpu(const struct cpumask *cpus, int cpu, int node)

Ah, this should be static of course.

> +{
> +	return cpumask_nth(cpu, cpus);
> +}
> +#endif	/* CONFIG_NUMA */
>  
>  #endif /* _LINUX_TOPOLOGY_H */
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 8739c2a5a54e..c8f56287de46 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -2067,6 +2067,48 @@ int sched_numa_find_closest(const struct cpumask *cpus, int cpu)
>  	return found;
>  }
>  
> +/*
> + * sched_numa_find_nth_cpu() - given the NUMA topology, find the Nth next cpu
> + *                             closest to @cpu from @cpumask.
> + * cpumask: cpumask to find a cpu from
> + * cpu: Nth cpu to find
> + *
> + * returns: cpu, or >= nr_cpu_ids when nothing found.
> + */
> +int sched_numa_find_nth_cpu(const struct cpumask *cpus, int cpu, int node)
> +{
> +	unsigned int first = 0, mid, last = sched_domains_numa_levels;
> +	struct cpumask ***masks;
> +	int w, ret = nr_cpu_ids;
> +
> +	rcu_read_lock();
> +	masks = rcu_dereference(sched_domains_numa_masks);
> +	if (!masks)
> +		goto out;
> +
> +	while (last >= first) {
> +		mid = (last + first) / 2;
> +
> +		if (cpumask_weight_and(cpus, masks[mid][node]) <= cpu) {
> +			first = mid + 1;
> +			continue;
> +		}
> +
> +		w = (mid == 0) ? 0 : cpumask_weight_and(cpus, masks[mid - 1][node]);
> +		if (w <= cpu)
> +			break;
> +
> +		last = mid - 1;
> +	}
> +
> +	ret = (mid == 0) ?
> +		cpumask_nth_and(cpu - w, cpus, masks[mid][node]) :
> +		cpumask_nth_and_andnot(cpu - w, cpus, masks[mid][node], masks[mid - 1][node]);
> +out:
> +	rcu_read_unlock();
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(sched_numa_find_nth_cpu);
>  #endif /* CONFIG_NUMA */
>  
>  static int __sdt_alloc(const struct cpumask *cpu_map)
> -- 
> 2.34.1