Re: [PATCH v2 00/17] sched/paravirt: Introduce cpu_preferred_mask and steal-driven vCPU backoff

Shrikanth Hegde
2026-04-07T19:50:37Z

> I will attach the irqbalance patch which detects the changes in this
> mask and re-adjusts the irq affinities. Series doesn't address when
> irqbalance=n. Assuming many distros have irqbalance=y by default.
>

Subject: [PATCH] irqbalance: Check for changes in cpu_preferred_mask

---
cputree.c | 28 +++++++++++++++++++++++++++-
irqbalance.c | 2 ++
irqbalance.h | 1 +
3 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/cputree.c b/cputree.c
index 9baa264..1db3422 100644
--- a/cputree.c
+++ b/cputree.c
@@ -56,6 +56,11 @@ cpumask_t banned_cpus;

cpumask_t cpu_online_map;

+/* This can dynamically change. If any change in mask detect
+ * and trigger a rebuild
+ */
+cpumask_t cpu_preferred_mask;
+
/*
it's convenient to have the complement of banned_cpus available so that
the AND operator can be used to mask out unwanted cpus
@@ -506,15 +511,36 @@ void clear_work_stats(void)
for_each_object(numa_nodes, clear_obj_stats, NULL);
}

+void parse_preferred_cpus(void)
+{
+ cpumask_t preferred;
+ char *path = NULL;
+
+ path = "/sys/devices/system/cpu/preferred";
+ cpus_clear(preferred);
+ process_one_line(path, get_mask_from_cpulist, &preferred);
+
+ /* Did anything change compared to earlier */
+ if (!cpus_equal(preferred, cpu_preferred_mask)) {
+ log(TO_CONSOLE, LOG_INFO, "cpu preferred mask changed\n");
+ need_rebuild = 1;
+ }
+
+ cpus_copy(cpu_preferred_mask, preferred);
+}

void parse_cpu_tree(void)
{
DIR *dir;
struct dirent *entry;
+ char buffer[4096];

setup_banned_cpus();

- cpus_complement(unbanned_cpus, banned_cpus);
+ cpus_andnot(unbanned_cpus, cpu_preferred_mask, banned_cpus);
+
+ cpumask_scnprintf(buffer, 4096, unbanned_cpus);
+ log(TO_CONSOLE, LOG_INFO, "Unbanned CPUs: %s\n", buffer);

dir = opendir("/sys/devices/system/cpu");
if (!dir)
diff --git a/irqbalance.c b/irqbalance.c
index f80244c..f3d46b8 100644
--- a/irqbalance.c
+++ b/irqbalance.c
@@ -229,6 +229,7 @@ static void parse_command_line(int argc, char **argv)
static void build_object_tree(void)
{
build_numa_node_list();
+ parse_preferred_cpus();
parse_cpu_tree();
rebuild_irq_db();
}
@@ -275,6 +276,7 @@ gboolean scan(gpointer data __attribute__((unused)))
log(TO_CONSOLE, LOG_INFO, "\n\n\n-----------------------------------------------------------------------------\n");
clear_work_stats();
parse_proc_interrupts();
+ parse_preferred_cpus();

/* cope with cpu hotplug -- detected during /proc/interrupts parsing */
diff --git a/irqbalance.h b/irqbalance.h
index 47e40cc..593b183 100644
--- a/irqbalance.h
+++ b/irqbalance.h
@@ -57,6 +57,7 @@ void migrate_irq_obj(struct topo_obj *from, struct topo_obj *to, struct irq_info
void activate_mappings(void);
void clear_cpu_tree(void);
void free_cpu_topo(gpointer data);
+extern void parse_preferred_cpus(void);
/*===================NEW BALANCER FUNCTIONS============================*/

/*
--
2.47.3