[Patch v1 1/3] lib: restricting cpumask_local_spread to only houskeeping CPUs

[Nitesh Narayan Lal]

From: Alex Belits <abelits@xxxxxxxxxxx>

The current implementation of cpumask_local_spread() does not
respect the isolated CPUs, i.e., even if a CPU has been isolated
for Real-Time task, it will return it to the caller for pinning
of its IRQ threads. Having these unwanted IRQ threads on an
isolated CPU adds up to a latency overhead.
This patch restricts the CPUs that are returned for spreading
IRQs only to the available housekeeping CPUs.

Signed-off-by: Alex Belits <abelits@xxxxxxxxxxx>
Signed-off-by: Nitesh Narayan Lal <nitesh@xxxxxxxxxx>
---
 lib/cpumask.c | 43 +++++++++++++++++++++++++------------------
 1 file changed, 25 insertions(+), 18 deletions(-)

diff --git a/lib/cpumask.c b/lib/cpumask.c
index fb22fb266f93..cc4311a8c079 100644
--- a/lib/cpumask.c
+++ b/lib/cpumask.c
@@ -6,6 +6,7 @@
 #include <linux/export.h>
 #include <linux/memblock.h>
 #include <linux/numa.h>
+#include <linux/sched/isolation.h>
 
 /**
  * cpumask_next - get the next cpu in a cpumask
@@ -205,28 +206,34 @@ void __init free_bootmem_cpumask_var(cpumask_var_t mask)
  */
 unsigned int cpumask_local_spread(unsigned int i, int node)
 {
-	int cpu;
+	int cpu, m, n, hk_flags;
+	const struct cpumask *mask;
 
+	hk_flags = HK_FLAG_DOMAIN | HK_FLAG_WQ;
+	mask = housekeeping_cpumask(hk_flags);
+	m = cpumask_weight(mask);
 	/* Wrap: we always want a cpu. */
-	i %= num_online_cpus();
+	n = i % m;
+	while (m-- > 0) {
+		if (node == NUMA_NO_NODE) {
+			for_each_cpu(cpu, mask)
+				if (n-- == 0)
+					return cpu;
+		} else {
+			/* NUMA first. */
+			for_each_cpu_and(cpu, cpumask_of_node(node), mask)
+				if (n-- == 0)
+					return cpu;
 
-	if (node == NUMA_NO_NODE) {
-		for_each_cpu(cpu, cpu_online_mask)
-			if (i-- == 0)
-				return cpu;
-	} else {
-		/* NUMA first. */
-		for_each_cpu_and(cpu, cpumask_of_node(node), cpu_online_mask)
-			if (i-- == 0)
-				return cpu;
+			for_each_cpu(cpu, mask) {
+				/* Skip NUMA nodes, done above. */
+				if (cpumask_test_cpu(cpu,
+						     cpumask_of_node(node)))
+					continue;
 
-		for_each_cpu(cpu, cpu_online_mask) {
-			/* Skip NUMA nodes, done above. */
-			if (cpumask_test_cpu(cpu, cpumask_of_node(node)))
-				continue;
-
-			if (i-- == 0)
-				return cpu;
+				if (n-- == 0)
+					return cpu;
+			}
 		}
 	}
 	BUG();
-- 
2.18.4