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]> 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
