timers/migration: Fix hotplug migrator selection target on asymetric capacity machines

When a top-level migrator is deactivated, either at CPU down hotplug time
or when a CPU is domain isolated, a new migrator is elected among the
available CPUs and woken up to take over the migration duty.

However that election must happen at the scope of a given hierarchy and not
globally, which the introduction of per-capacity hierarchies failed to
handle.

As a result a given hierarchy may end up without migrator to handle global
timers.

Fix it by making sure that the new migrator belongs to the same hierarchy
as the outgoing CPU.

Fixes: 098cbaad8e57 ("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/20260519220926.63437-2-frederic@kernel.org

diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c
index 25e3c563eb748586231b65c9be15c142215e0403..8032b0044f44a288320f8846ebeee5c331875a83 100644 (file)
--- a/kernel/time/timer_migration.c
+++ b/kernel/time/timer_migration.c
@@ -1464,6 +1464,18 @@ static long tmigr_trigger_active(void *unused)
        return 0;
 }
 
+static struct tmigr_hierarchy *__tmigr_get_hierarchy(unsigned int capacity)
+{
+       struct tmigr_hierarchy *iter;
+
+       list_for_each_entry(iter, &tmigr_hierarchy_list, node) {
+               if (iter->capacity == capacity)
+                       return iter;
+       }
+
+       return NULL;
+}
+
 static int tmigr_clear_cpu_available(unsigned int cpu)
 {
        struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
@@ -1488,8 +1500,21 @@ static int tmigr_clear_cpu_available(unsigned int cpu)
        }
 
        if (firstexp != KTIME_MAX) {
-               migrator = cpumask_any(tmigr_available_cpumask);
-               work_on_cpu(migrator, tmigr_trigger_active, NULL);
+               struct tmigr_hierarchy *hier = __tmigr_get_hierarchy(arch_scale_cpu_capacity(cpu));
+
+               if (WARN_ON_ONCE(!hier))
+                       return -EINVAL;
+
+               migrator = cpumask_any_and(tmigr_available_cpumask, hier->cpumask);
+               if (migrator < nr_cpu_ids) {
+                       work_on_cpu(migrator, tmigr_trigger_active, NULL);
+               } else {
+                       /*
+                        * If deactivation returned an expiration, it belongs to an available
+                        * nohz CPU in the hierarchy.
+                        */
+                       WARN_ONCE(1, "Expected available CPU in the hierarchy\n");
+               }
        }
 
        return 0;
@@ -1915,12 +1940,9 @@ out:
 
 static struct tmigr_hierarchy *tmigr_get_hierarchy(unsigned int capacity)
 {
-       struct tmigr_hierarchy *hier = NULL, *iter;
+       struct tmigr_hierarchy *hier;
 
-       list_for_each_entry(iter, &tmigr_hierarchy_list, node) {
-               if (iter->capacity == capacity)
-                       hier = iter;
-       }
+       hier = __tmigr_get_hierarchy(capacity);
 
        if (hier)
                return hier;
@@ -1978,9 +2000,9 @@ 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));
-       if (IS_ERR(hier))
-               return PTR_ERR(hier);
+       hier = __tmigr_get_hierarchy(arch_scale_cpu_capacity(cpu));
+       if (WARN_ON_ONCE(!hier))
+               return -EINVAL;
 
        return tmigr_connect_old_root(hier, cpu, old_root, true);
 }