Keep track of the CPUs available for timer migration in a cpumask. This
prepares the ground to generalise the concept of unavailable CPUs.

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
---
 kernel/time/timer_migration.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c
index d50514062a06..b8f07e4f59a6 100644
--- a/kernel/time/timer_migration.c
+++ b/kernel/time/timer_migration.c
@@ -424,6 +424,12 @@ static struct tmigr_group *tmigr_root;
 
 static DEFINE_PER_CPU(struct tmigr_cpu, tmigr_cpu);
 
+/*
+ * CPUs available for timer migration.
+ * Protected by cpuset_mutex (with cpus_read_lock held) or cpus_write_lock.
+ */
+static cpumask_var_t tmigr_available_cpumask;
+
 #define TMIGR_NONE	0xFF
 #define BIT_CNT		8
 
@@ -1433,6 +1439,7 @@ static int tmigr_clear_cpu_available(unsigned int cpu)
 	int migrator;
 	u64 firstexp;
 
+	cpumask_clear_cpu(cpu, tmigr_available_cpumask);
 	raw_spin_lock_irq(&tmc->lock);
 	tmc->available = false;
 	WRITE_ONCE(tmc->wakeup, KTIME_MAX);
@@ -1446,7 +1453,7 @@ static int tmigr_clear_cpu_available(unsigned int cpu)
 	raw_spin_unlock_irq(&tmc->lock);
 
 	if (firstexp != KTIME_MAX) {
-		migrator = cpumask_any_but(cpu_online_mask, cpu);
+		migrator = cpumask_any(tmigr_available_cpumask);
 		work_on_cpu(migrator, tmigr_trigger_active, NULL);
 	}
 
@@ -1461,6 +1468,7 @@ static int tmigr_set_cpu_available(unsigned int cpu)
 	if (WARN_ON_ONCE(!tmc->tmgroup))
 		return -EINVAL;
 
+	cpumask_set_cpu(cpu, tmigr_available_cpumask);
 	raw_spin_lock_irq(&tmc->lock);
 	trace_tmigr_cpu_available(tmc);
 	tmc->idle = timer_base_is_idle();
@@ -1805,6 +1813,11 @@ static int __init tmigr_init(void)
 	if (ncpus == 1)
 		return 0;
 
+	if (!zalloc_cpumask_var(&tmigr_available_cpumask, GFP_KERNEL)) {
+		ret = -ENOMEM;
+		goto err;
+	}
+
 	/*
 	 * Calculate the required hierarchy levels. Unfortunately there is no
 	 * reliable information available, unless all possible CPUs have been
-- 
2.51.1

On Thu, Nov 13 2025 at 09:33, Gabriele Monaco wrote:

Can we please consistently have 'timers/migration:' as prefix like the
other existing commits for this series? 

That allows to write this subject line in a coherent way:

     timers/migration: Provide and use cpumask for available CPUs

or something like that.

Why did I waste time for documenting all of this?

   https://www.kernel.org/doc/html/latest/process/maintainer-tip.html#patch-subject

   "‘git log path/to/file’ should give you a reasonable hint in most cases."

Is it really that hard?

> Keep track of the CPUs available for timer migration in a cpumask. This
> prepares the ground to generalise the concept of unavailable CPUs.
>
> Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
> Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>

Aside of the nit above:

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>