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Message-ID: <20250915123738.GD3245006@noisy.programming.kicks-ass.net>
Date: Mon, 15 Sep 2025 14:37:38 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Tim Chen <tim.c.chen@...ux.intel.com>
Cc: Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Valentin Schneider <vschneid@...hat.com>,
Tim Chen <tim.c.chen@...el.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Libo Chen <libo.chen@...cle.com>,
Abel Wu <wuyun.abel@...edance.com>, Len Brown <len.brown@...el.com>,
linux-kernel@...r.kernel.org, Chen Yu <yu.c.chen@...el.com>,
K Prateek Nayak <kprateek.nayak@....com>,
"Gautham R . Shenoy" <gautham.shenoy@....com>,
Zhao Liu <zhao1.liu@...el.com>,
Vinicius Costa Gomes <vinicius.gomes@...el.com>,
Arjan Van De Ven <arjan.van.de.ven@...el.com>
Subject: Re: [PATCH v3 1/2] sched: Create architecture specific sched domain
distances
On Thu, Sep 11, 2025 at 11:30:56AM -0700, Tim Chen wrote:
> Allow architecture specific sched domain NUMA distances that can be
> modified from NUMA node distances for the purpose of building NUMA
> sched domains.
>
> The actual NUMA distances are kept separately. This allows for NUMA
> domain levels modification when building sched domains for specific
> architectures.
>
> Consolidate the recording of unique NUMA distances in an array to
> sched_record_numa_dist() so the function can be reused to record NUMA
> distances when the NUMA distance metric is changed.
>
> No functional change if there's no arch specific NUMA distances
> are being defined.
Keeping both metrics side-by-side is confusing -- and not very well
justified by the above.
Is there any appreciable benefit to mixing the two like this?
>
> Co-developed-by: Vinicius Costa Gomes <vinicius.gomes@...el.com>
> Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@...el.com>
> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
> ---
> include/linux/sched/topology.h | 2 +
> kernel/sched/topology.c | 114 ++++++++++++++++++++++++++++-----
> 2 files changed, 99 insertions(+), 17 deletions(-)
>
> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> index 5263746b63e8..4f58e78ca52e 100644
> --- a/include/linux/sched/topology.h
> +++ b/include/linux/sched/topology.h
> @@ -59,6 +59,8 @@ static inline int cpu_numa_flags(void)
> #endif
>
> extern int arch_asym_cpu_priority(int cpu);
> +extern int arch_sched_node_distance(int from, int to);
> +extern int sched_avg_remote_numa_distance;
>
> struct sched_domain_attr {
> int relax_domain_level;
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 977e133bb8a4..6c0ff62322cb 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -1591,10 +1591,13 @@ static void claim_allocations(int cpu, struct sched_domain *sd)
> enum numa_topology_type sched_numa_topology_type;
>
> static int sched_domains_numa_levels;
> +static int sched_numa_node_levels;
> static int sched_domains_curr_level;
>
> int sched_max_numa_distance;
> +int sched_avg_remote_numa_distance;
> static int *sched_domains_numa_distance;
> +static int *sched_numa_node_distance;
> static struct cpumask ***sched_domains_numa_masks;
> #endif /* CONFIG_NUMA */
>
> @@ -1808,10 +1811,10 @@ bool find_numa_distance(int distance)
> return true;
>
> rcu_read_lock();
> - distances = rcu_dereference(sched_domains_numa_distance);
> + distances = rcu_dereference(sched_numa_node_distance);
> if (!distances)
> goto unlock;
> - for (i = 0; i < sched_domains_numa_levels; i++) {
> + for (i = 0; i < sched_numa_node_levels; i++) {
> if (distances[i] == distance) {
> found = true;
> break;
I'm assuming (because its not actually stated anywhere) that
sched_numa_$FOO is based on the SLIT table, while sched_domain_$FOO is
the modified thing.
And you're saying it makes a significant difference to
preferred_group_nid()?
> +static int sched_record_numa_dist(int offline_node, int (*n_dist)(int, int),
> + int **dist, int *levels)
> +
That's a coding style fail; use cino=(0:0.
> {
> - struct sched_domain_topology_level *tl;
> unsigned long *distance_map;
> int nr_levels = 0;
> int i, j;
> int *distances;
> - struct cpumask ***masks;
>
> /*
> * O(nr_nodes^2) de-duplicating selection sort -- in order to find the
> @@ -1902,17 +1923,17 @@ void sched_init_numa(int offline_node)
> */
> distance_map = bitmap_alloc(NR_DISTANCE_VALUES, GFP_KERNEL);
> if (!distance_map)
> - return;
> + return -ENOMEM;
>
> bitmap_zero(distance_map, NR_DISTANCE_VALUES);
> for_each_cpu_node_but(i, offline_node) {
> for_each_cpu_node_but(j, offline_node) {
> - int distance = node_distance(i, j);
> + int distance = n_dist(i, j);
>
> if (distance < LOCAL_DISTANCE || distance >= NR_DISTANCE_VALUES) {
> sched_numa_warn("Invalid distance value range");
> bitmap_free(distance_map);
> - return;
> + return -EINVAL;
> }
>
> bitmap_set(distance_map, distance, 1);
> @@ -1927,17 +1948,66 @@ void sched_init_numa(int offline_node)
> distances = kcalloc(nr_levels, sizeof(int), GFP_KERNEL);
> if (!distances) {
> bitmap_free(distance_map);
> - return;
> + return -ENOMEM;
> }
> -
> for (i = 0, j = 0; i < nr_levels; i++, j++) {
> j = find_next_bit(distance_map, NR_DISTANCE_VALUES, j);
> distances[i] = j;
> }
> - rcu_assign_pointer(sched_domains_numa_distance, distances);
> + *dist = distances;
> + *levels = nr_levels;
>
> bitmap_free(distance_map);
>
> + return 0;
> +}
> +
> +static int avg_remote_numa_distance(int offline_node)
> +{
> + int i, j;
> + int distance, nr_remote = 0, total_distance = 0;
> +
> + for_each_cpu_node_but(i, offline_node) {
> + for_each_cpu_node_but(j, offline_node) {
> + distance = node_distance(i, j);
> +
> + if (distance >= REMOTE_DISTANCE) {
> + nr_remote++;
> + total_distance += distance;
> + }
> + }
> + }
> + if (nr_remote)
> + return total_distance / nr_remote;
> + else
> + return REMOTE_DISTANCE;
> +}
> +
> +void sched_init_numa(int offline_node)
> +{
> + struct sched_domain_topology_level *tl;
> + int nr_levels, nr_node_levels;
> + int i, j;
> + int *distances, *domain_distances;
> + struct cpumask ***masks;
> +
> + if (sched_record_numa_dist(offline_node, numa_node_dist, &distances,
> + &nr_node_levels))
> + return;
> +
> + WRITE_ONCE(sched_avg_remote_numa_distance,
> + avg_remote_numa_distance(offline_node));
What is the point of all this? sched_avg_remote_numa_distance isn't
actually used anywhere. I'm thinking it doesn't want to be in this patch
at the very least.
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