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

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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: 098cbaad8e ("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
This commit is contained in:
Frederic Weisbecker 2026-05-20 00:09:25 +02:00 committed by Thomas Gleixner
parent 180a232ea7
commit e4a70f5fbd
1 changed files with 32 additions and 10 deletions
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42
kernel/time/timer_migration.c
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@ -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 @@ static int tmigr_setup_groups(struct tmigr_hierarchy *hier, unsigned int cpu,
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);
}