timers/migration: Deactivate per-capacity hierarchies under nohz_full

NOHZ_FULL CPUs global timers are guaranteed to be handled by the timekeeper
CPU, which never stops its tick and therefore remains active in the
hierarchy.

But since the introduction of per-capacity hierarchies, this guarantee is
broken because the timekeeper may not belong to the same hierarchy as all
the NOHZ_FULL CPUs.

Fix it with simply turning off capacity awareness when NOHZ_FULL is
running and force a single hierarchy. NOHZ_FULL is not exactly optimized
powerwise anyway.

Fixes: 098cbaa ("timers/migration: Split per-capacity hierarchies")
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260519220926.63437-3-frederic@kernel.org

Author: Frederic Weisbecker

kernel/time/timer_migration.c

@@ -1464,8 +1464,24 @@ static long tmigr_trigger_active(void *unused)
 	return 0;
 }
 
-static struct tmigr_hierarchy *__tmigr_get_hierarchy(unsigned int capacity)
+static unsigned int tmigr_get_capacity(int cpu)
 {
+	/*
+	 * nohz_full CPUs need to make sure there is always an available (online)
+	 * and never idle migrator to handle all their global timers. That duty
+	 * is served by the timekeeper which then never stops its tick. But the
+	 * timekeeper must then belong to the same hierarchy as all the nohz_full
+	 * CPUs. Simply turn off capacity awareness when nohz_full is running.
+	 */
+	if (tick_nohz_full_enabled())
+		return SCHED_CAPACITY_SCALE;
+	else
+		return arch_scale_cpu_capacity(cpu);
+}
+
+static struct tmigr_hierarchy *__tmigr_get_hierarchy(int cpu)
+{
+	unsigned int capacity = tmigr_get_capacity(cpu);
 	struct tmigr_hierarchy *iter;
 
 	list_for_each_entry(iter, &tmigr_hierarchy_list, node) {
@@ -1500,7 +1516,7 @@ static int tmigr_clear_cpu_available(unsigned int cpu)
 	}
 
 	if (firstexp != KTIME_MAX) {
-		struct tmigr_hierarchy *hier = __tmigr_get_hierarchy(arch_scale_cpu_capacity(cpu));
+		struct tmigr_hierarchy *hier = __tmigr_get_hierarchy(cpu);
 
 		if (WARN_ON_ONCE(!hier))
 			return -EINVAL;
@@ -1938,11 +1954,11 @@ static int tmigr_setup_groups(struct tmigr_hierarchy *hier, unsigned int cpu,
 	return err;
 }
 
-static struct tmigr_hierarchy *tmigr_get_hierarchy(unsigned int capacity)
+static struct tmigr_hierarchy *tmigr_get_hierarchy(int cpu)
 {
 	struct tmigr_hierarchy *hier;
 
-	hier = __tmigr_get_hierarchy(capacity);
+	hier = __tmigr_get_hierarchy(cpu);
 
 	if (hier)
 		return hier;
@@ -1962,7 +1978,7 @@ static struct tmigr_hierarchy *tmigr_get_hierarchy(unsigned int capacity)
 	for (int i = 0; i < tmigr_hierarchy_levels; i++)
 		INIT_LIST_HEAD(&hier->level_list[i]);
 
-	hier->capacity = capacity;
+	hier->capacity = tmigr_get_capacity(cpu);
 	list_add_tail(&hier->node, &tmigr_hierarchy_list);
 
 	return hier;
@@ -2000,7 +2016,7 @@ static long connect_old_root_work(void *arg)
 	struct tmigr_hierarchy *hier;
 	int cpu = smp_processor_id();
 
-	hier = __tmigr_get_hierarchy(arch_scale_cpu_capacity(cpu));
+	hier = __tmigr_get_hierarchy(cpu);
 	if (WARN_ON_ONCE(!hier))
 		return -EINVAL;
 
@@ -2016,7 +2032,7 @@ static int tmigr_add_cpu(unsigned int cpu)
 
 	guard(mutex)(&tmigr_mutex);
 
-	hier = tmigr_get_hierarchy(arch_scale_cpu_capacity(cpu));
+	hier = tmigr_get_hierarchy(cpu);
 	if (IS_ERR(hier))
 		return PTR_ERR(hier);