[PATCH v2 1/2] cpuhp: Optimize SMT switch operation by batching lock acquisition
From: Vishal Chourasia
Date: Mon Feb 16 2026 - 07:22:08 EST
Bulk CPU hotplug operations, such as an SMT switch operation, requires
hotplugging multiple CPUs. The current implementation takes
cpus_write_lock() for each individual CPU, causing multiple slow grace
period requests.
Introduce cpu_up_locked() and cpu_down_locked() that assume the caller
already holds cpus_write_lock(). The cpuhp_smt_enable() and
cpuhp_smt_disable() functions are updated to hold the lock once around
the entire loop, rather than for each individual CPU.
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Vishal Chourasia <vishalc@xxxxxxxxxxxxx>
---
kernel/cpu.c | 73 ++++++++++++++++++++++++++++++++++++++--------------
1 file changed, 53 insertions(+), 20 deletions(-)
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 01968a5c4a16..edaa37419036 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1400,8 +1400,8 @@ static int cpuhp_down_callbacks(unsigned int cpu, struct cpuhp_cpu_state *st,
return ret;
}
-/* Requires cpu_add_remove_lock to be held */
-static int __ref _cpu_down(unsigned int cpu, int tasks_frozen,
+/* Requires cpu_add_remove_lock and cpus_write_lock to be held */
+static int __ref cpu_down_locked(unsigned int cpu, int tasks_frozen,
enum cpuhp_state target)
{
struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
@@ -1413,7 +1413,7 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen,
if (!cpu_present(cpu))
return -EINVAL;
- cpus_write_lock();
+ lockdep_assert_cpus_held();
/*
* Keep at least one housekeeping cpu onlined to avoid generating
@@ -1421,8 +1421,7 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen,
*/
if (cpumask_any_and(cpu_online_mask,
housekeeping_cpumask(HK_TYPE_DOMAIN)) >= nr_cpu_ids) {
- ret = -EBUSY;
- goto out;
+ return -EBUSY;
}
cpuhp_tasks_frozen = tasks_frozen;
@@ -1440,14 +1439,14 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen,
* return the error code..
*/
if (ret)
- goto out;
+ return ret;
/*
* We might have stopped still in the range of the AP hotplug
* thread. Nothing to do anymore.
*/
if (st->state > CPUHP_TEARDOWN_CPU)
- goto out;
+ return 0;
st->target = target;
}
@@ -1464,8 +1463,17 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen,
WARN(1, "DEAD callback error for CPU%d", cpu);
}
}
+ return ret;
+}
-out:
+static int __ref _cpu_down(unsigned int cpu, int tasks_frozen,
+ enum cpuhp_state target)
+{
+
+ int ret;
+
+ cpus_write_lock();
+ ret = cpu_down_locked(cpu, tasks_frozen, target);
cpus_write_unlock();
arch_smt_update();
return ret;
@@ -1613,18 +1621,18 @@ void cpuhp_online_idle(enum cpuhp_state state)
complete_ap_thread(st, true);
}
-/* Requires cpu_add_remove_lock to be held */
-static int _cpu_up(unsigned int cpu, int tasks_frozen, enum cpuhp_state target)
+/* Requires cpu_add_remove_lock and cpus_write_lock to be held. */
+static int cpu_up_locked(unsigned int cpu, int tasks_frozen,
+ enum cpuhp_state target)
{
struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
struct task_struct *idle;
int ret = 0;
- cpus_write_lock();
+ lockdep_assert_cpus_held();
if (!cpu_present(cpu)) {
- ret = -EINVAL;
- goto out;
+ return -EINVAL;
}
/*
@@ -1632,14 +1640,13 @@ static int _cpu_up(unsigned int cpu, int tasks_frozen, enum cpuhp_state target)
* caller. Nothing to do.
*/
if (st->state >= target)
- goto out;
+ return 0;
if (st->state == CPUHP_OFFLINE) {
/* Let it fail before we try to bring the cpu up */
idle = idle_thread_get(cpu);
if (IS_ERR(idle)) {
- ret = PTR_ERR(idle);
- goto out;
+ return PTR_ERR(idle);
}
/*
@@ -1663,7 +1670,7 @@ static int _cpu_up(unsigned int cpu, int tasks_frozen, enum cpuhp_state target)
* return the error code..
*/
if (ret)
- goto out;
+ return ret;
}
/*
@@ -1673,7 +1680,16 @@ static int _cpu_up(unsigned int cpu, int tasks_frozen, enum cpuhp_state target)
*/
target = min((int)target, CPUHP_BRINGUP_CPU);
ret = cpuhp_up_callbacks(cpu, st, target);
-out:
+ return ret;
+}
+
+/* Requires cpu_add_remove_lock to be held */
+static int _cpu_up(unsigned int cpu, int tasks_frozen, enum cpuhp_state target)
+{
+ int ret;
+
+ cpus_write_lock();
+ ret = cpu_up_locked(cpu, tasks_frozen, target);
cpus_write_unlock();
arch_smt_update();
return ret;
@@ -2659,6 +2675,16 @@ int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval)
int cpu, ret = 0;
cpu_maps_update_begin();
+ if (cpu_hotplug_offline_disabled) {
+ ret = -EOPNOTSUPP;
+ goto out;
+ }
+ if (cpu_hotplug_disabled) {
+ ret = -EBUSY;
+ goto out;
+ }
+ /* Hold cpus_write_lock() for entire batch operation. */
+ cpus_write_lock();
for_each_online_cpu(cpu) {
if (topology_is_primary_thread(cpu))
continue;
@@ -2668,7 +2694,7 @@ int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval)
*/
if (ctrlval == CPU_SMT_ENABLED && cpu_smt_thread_allowed(cpu))
continue;
- ret = cpu_down_maps_locked(cpu, CPUHP_OFFLINE);
+ ret = cpu_down_locked(cpu, 0, CPUHP_OFFLINE);
if (ret)
break;
/*
@@ -2688,6 +2714,9 @@ int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval)
}
if (!ret)
cpu_smt_control = ctrlval;
+ cpus_write_unlock();
+ arch_smt_update();
+out:
cpu_maps_update_done();
return ret;
}
@@ -2705,6 +2734,8 @@ int cpuhp_smt_enable(void)
int cpu, ret = 0;
cpu_maps_update_begin();
+ /* Hold cpus_write_lock() for entire batch operation. */
+ cpus_write_lock();
cpu_smt_control = CPU_SMT_ENABLED;
for_each_present_cpu(cpu) {
/* Skip online CPUs and CPUs on offline nodes */
@@ -2712,12 +2743,14 @@ int cpuhp_smt_enable(void)
continue;
if (!cpu_smt_thread_allowed(cpu) || !topology_is_core_online(cpu))
continue;
- ret = _cpu_up(cpu, 0, CPUHP_ONLINE);
+ ret = cpu_up_locked(cpu, 0, CPUHP_ONLINE);
if (ret)
break;
/* See comment in cpuhp_smt_disable() */
cpuhp_online_cpu_device(cpu);
}
+ cpus_write_unlock();
+ arch_smt_update();
cpu_maps_update_done();
return ret;
}
--
2.53.0