On Mon, Feb 16, 2026 at 05:49:28PM +0530, Vishal Chourasia wrote: > 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 <[email protected]> The code hositing the cpus_write_lock up in the cpuhp_smt_enable() was provided by Joel [1]. Thanks Joel.
I missed adding an appropriate tag for it. Originally-by: Joel Fernandes <[email protected]> [1] https://lore.kernel.org/all/20260119051835.GA696111@joelbox2/ > Signed-off-by: Vishal Chourasia <[email protected]> > --- > 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 >
