[PATCH v3 4/5] cpumask: improve on cpumask_local_spread() locality

[Yury Norov]

Switch cpumask_local_spread() to use newly added sched_numa_find_nth_cpu(),
which takes into account distances to each node in the system.

For the following NUMA configuration:

root@debian:~# numactl -H
available: 4 nodes (0-3)
node 0 cpus: 0 1 2 3
node 0 size: 3869 MB
node 0 free: 3740 MB
node 1 cpus: 4 5
node 1 size: 1969 MB
node 1 free: 1937 MB
node 2 cpus: 6 7
node 2 size: 1967 MB
node 2 free: 1873 MB
node 3 cpus: 8 9 10 11 12 13 14 15
node 3 size: 7842 MB
node 3 free: 7723 MB
node distances:
node   0   1   2   3
  0:  10  50  30  70
  1:  50  10  70  30
  2:  30  70  10  50
  3:  70  30  50  10

The new cpumask_local_spread() traverses cpus for each node like this:

node 0:   0   1   2   3   6   7   4   5   8   9  10  11  12  13  14  15
node 1:   4   5   8   9  10  11  12  13  14  15   0   1   2   3   6   7
node 2:   6   7   0   1   2   3   8   9  10  11  12  13  14  15   4   5
node 3:   8   9  10  11  12  13  14  15   4   5   6   7   0   1   2   3

Signed-off-by: Yury Norov <yury.norov@xxxxxxxxx>
---
 lib/cpumask.c | 12 ++----------
 1 file changed, 2 insertions(+), 10 deletions(-)

diff --git a/lib/cpumask.c b/lib/cpumask.c
index c7c392514fd3..255974cd6734 100644
--- a/lib/cpumask.c
+++ b/lib/cpumask.c
@@ -110,7 +110,7 @@ void __init free_bootmem_cpumask_var(cpumask_var_t mask)
 #endif
 
 /**
- * cpumask_local_spread - select the i'th cpu with local numa cpu's first
+ * cpumask_local_spread - select the i'th cpu based on NUMA distances
  * @i: index number
  * @node: local numa_node
  *
@@ -132,15 +132,7 @@ unsigned int cpumask_local_spread(unsigned int i, int node)
 		if (cpu < nr_cpu_ids)
 			return cpu;
 	} else {
-		/* NUMA first. */
-		cpu = cpumask_nth_and(i, cpu_online_mask, cpumask_of_node(node));
-		if (cpu < nr_cpu_ids)
-			return cpu;
-
-		i -= cpumask_weight_and(cpu_online_mask, cpumask_of_node(node));
-
-		/* Skip NUMA nodes, done above. */
-		cpu = cpumask_nth_andnot(i, cpu_online_mask, cpumask_of_node(node));
+		cpu = sched_numa_find_nth_cpu(cpu_online_mask, i, node);
 		if (cpu < nr_cpu_ids)
 			return cpu;
 	}
-- 
2.34.1