From d221938c049f4845da13c8593132595a6b9222a8 Mon Sep 17 00:00:00 2001 From: Gautham R Shenoy Date: Fri, 25 Jan 2008 21:08:01 +0100 Subject: [PATCH] cpu-hotplug: refcount based cpu hotplug This patch implements a Refcount + Waitqueue based model for cpu-hotplug. Now, a thread which wants to prevent cpu-hotplug, will bump up a global refcount and the thread which wants to perform a cpu-hotplug operation will block till the global refcount goes to zero. The readers, if any, during an ongoing cpu-hotplug operation are blocked until the cpu-hotplug operation is over. Signed-off-by: Gautham R Shenoy Signed-off-by: Paul Jackson [For !CONFIG_HOTPLUG_CPU ] Signed-off-by: Ingo Molnar --- include/linux/cpu.h | 3 + init/main.c | 1 + kernel/cpu.c | 152 ++++++++++++++++++++++++++++++++------------ 3 files changed, 115 insertions(+), 41 deletions(-) diff --git a/include/linux/cpu.h b/include/linux/cpu.h index 92f2029a34f..a40247e4d46 100644 --- a/include/linux/cpu.h +++ b/include/linux/cpu.h @@ -83,6 +83,9 @@ static inline void unregister_cpu_notifier(struct notifier_block *nb) #endif /* CONFIG_SMP */ extern struct sysdev_class cpu_sysdev_class; +extern void cpu_hotplug_init(void); +extern void cpu_maps_update_begin(void); +extern void cpu_maps_update_done(void); #ifdef CONFIG_HOTPLUG_CPU /* Stop CPUs going up and down. */ diff --git a/init/main.c b/init/main.c index 80b04b6c515..f287ca5862b 100644 --- a/init/main.c +++ b/init/main.c @@ -607,6 +607,7 @@ asmlinkage void __init start_kernel(void) vfs_caches_init_early(); cpuset_init_early(); mem_init(); + cpu_hotplug_init(); kmem_cache_init(); setup_per_cpu_pageset(); numa_policy_init(); diff --git a/kernel/cpu.c b/kernel/cpu.c index 6b3a0c15144..656dc3fcbba 100644 --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -15,9 +15,8 @@ #include #include -/* This protects CPUs going up and down... */ +/* Serializes the updates to cpu_online_map, cpu_present_map */ static DEFINE_MUTEX(cpu_add_remove_lock); -static DEFINE_MUTEX(cpu_bitmask_lock); static __cpuinitdata RAW_NOTIFIER_HEAD(cpu_chain); @@ -26,52 +25,123 @@ static __cpuinitdata RAW_NOTIFIER_HEAD(cpu_chain); */ static int cpu_hotplug_disabled; -#ifdef CONFIG_HOTPLUG_CPU +static struct { + struct task_struct *active_writer; + struct mutex lock; /* Synchronizes accesses to refcount, */ + /* + * Also blocks the new readers during + * an ongoing cpu hotplug operation. + */ + int refcount; + wait_queue_head_t writer_queue; +} cpu_hotplug; -/* Crappy recursive lock-takers in cpufreq! Complain loudly about idiots */ -static struct task_struct *recursive; -static int recursive_depth; +#define writer_exists() (cpu_hotplug.active_writer != NULL) + +void __init cpu_hotplug_init(void) +{ + cpu_hotplug.active_writer = NULL; + mutex_init(&cpu_hotplug.lock); + cpu_hotplug.refcount = 0; + init_waitqueue_head(&cpu_hotplug.writer_queue); +} + +#ifdef CONFIG_HOTPLUG_CPU void lock_cpu_hotplug(void) { - struct task_struct *tsk = current; - - if (tsk == recursive) { - static int warnings = 10; - if (warnings) { - printk(KERN_ERR "Lukewarm IQ detected in hotplug locking\n"); - WARN_ON(1); - warnings--; - } - recursive_depth++; + might_sleep(); + if (cpu_hotplug.active_writer == current) return; - } - mutex_lock(&cpu_bitmask_lock); - recursive = tsk; + mutex_lock(&cpu_hotplug.lock); + cpu_hotplug.refcount++; + mutex_unlock(&cpu_hotplug.lock); + } EXPORT_SYMBOL_GPL(lock_cpu_hotplug); void unlock_cpu_hotplug(void) { - WARN_ON(recursive != current); - if (recursive_depth) { - recursive_depth--; + if (cpu_hotplug.active_writer == current) return; - } - recursive = NULL; - mutex_unlock(&cpu_bitmask_lock); + mutex_lock(&cpu_hotplug.lock); + cpu_hotplug.refcount--; + + if (unlikely(writer_exists()) && !cpu_hotplug.refcount) + wake_up(&cpu_hotplug.writer_queue); + + mutex_unlock(&cpu_hotplug.lock); + } EXPORT_SYMBOL_GPL(unlock_cpu_hotplug); #endif /* CONFIG_HOTPLUG_CPU */ +/* + * The following two API's must be used when attempting + * to serialize the updates to cpu_online_map, cpu_present_map. + */ +void cpu_maps_update_begin(void) +{ + mutex_lock(&cpu_add_remove_lock); +} + +void cpu_maps_update_done(void) +{ + mutex_unlock(&cpu_add_remove_lock); +} + +/* + * This ensures that the hotplug operation can begin only when the + * refcount goes to zero. + * + * Note that during a cpu-hotplug operation, the new readers, if any, + * will be blocked by the cpu_hotplug.lock + * + * Since cpu_maps_update_begin is always called after invoking + * cpu_maps_update_begin, we can be sure that only one writer is active. + * + * Note that theoretically, there is a possibility of a livelock: + * - Refcount goes to zero, last reader wakes up the sleeping + * writer. + * - Last reader unlocks the cpu_hotplug.lock. + * - A new reader arrives at this moment, bumps up the refcount. + * - The writer acquires the cpu_hotplug.lock finds the refcount + * non zero and goes to sleep again. + * + * However, this is very difficult to achieve in practice since + * lock_cpu_hotplug() not an api which is called all that often. + * + */ +static void cpu_hotplug_begin(void) +{ + DECLARE_WAITQUEUE(wait, current); + + mutex_lock(&cpu_hotplug.lock); + + cpu_hotplug.active_writer = current; + add_wait_queue_exclusive(&cpu_hotplug.writer_queue, &wait); + while (cpu_hotplug.refcount) { + set_current_state(TASK_UNINTERRUPTIBLE); + mutex_unlock(&cpu_hotplug.lock); + schedule(); + mutex_lock(&cpu_hotplug.lock); + } + remove_wait_queue_locked(&cpu_hotplug.writer_queue, &wait); +} + +static void cpu_hotplug_done(void) +{ + cpu_hotplug.active_writer = NULL; + mutex_unlock(&cpu_hotplug.lock); +} /* Need to know about CPUs going up/down? */ int __cpuinit register_cpu_notifier(struct notifier_block *nb) { int ret; - mutex_lock(&cpu_add_remove_lock); + cpu_maps_update_begin(); ret = raw_notifier_chain_register(&cpu_chain, nb); - mutex_unlock(&cpu_add_remove_lock); + cpu_maps_update_done(); return ret; } @@ -81,9 +151,9 @@ EXPORT_SYMBOL(register_cpu_notifier); void unregister_cpu_notifier(struct notifier_block *nb) { - mutex_lock(&cpu_add_remove_lock); + cpu_maps_update_begin(); raw_notifier_chain_unregister(&cpu_chain, nb); - mutex_unlock(&cpu_add_remove_lock); + cpu_maps_update_done(); } EXPORT_SYMBOL(unregister_cpu_notifier); @@ -147,6 +217,7 @@ static int _cpu_down(unsigned int cpu, int tasks_frozen) if (!cpu_online(cpu)) return -EINVAL; + cpu_hotplug_begin(); raw_notifier_call_chain(&cpu_chain, CPU_LOCK_ACQUIRE, hcpu); err = __raw_notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE | mod, hcpu, -1, &nr_calls); @@ -166,9 +237,7 @@ static int _cpu_down(unsigned int cpu, int tasks_frozen) cpu_clear(cpu, tmp); set_cpus_allowed(current, tmp); - mutex_lock(&cpu_bitmask_lock); p = __stop_machine_run(take_cpu_down, &tcd_param, cpu); - mutex_unlock(&cpu_bitmask_lock); if (IS_ERR(p) || cpu_online(cpu)) { /* CPU didn't die: tell everyone. Can't complain. */ @@ -203,6 +272,7 @@ out_allowed: set_cpus_allowed(current, old_allowed); out_release: raw_notifier_call_chain(&cpu_chain, CPU_LOCK_RELEASE, hcpu); + cpu_hotplug_done(); return err; } @@ -210,13 +280,13 @@ int cpu_down(unsigned int cpu) { int err = 0; - mutex_lock(&cpu_add_remove_lock); + cpu_maps_update_begin(); if (cpu_hotplug_disabled) err = -EBUSY; else err = _cpu_down(cpu, 0); - mutex_unlock(&cpu_add_remove_lock); + cpu_maps_update_done(); return err; } #endif /*CONFIG_HOTPLUG_CPU*/ @@ -231,6 +301,7 @@ static int __cpuinit _cpu_up(unsigned int cpu, int tasks_frozen) if (cpu_online(cpu) || !cpu_present(cpu)) return -EINVAL; + cpu_hotplug_begin(); raw_notifier_call_chain(&cpu_chain, CPU_LOCK_ACQUIRE, hcpu); ret = __raw_notifier_call_chain(&cpu_chain, CPU_UP_PREPARE | mod, hcpu, -1, &nr_calls); @@ -243,9 +314,7 @@ static int __cpuinit _cpu_up(unsigned int cpu, int tasks_frozen) } /* Arch-specific enabling code. */ - mutex_lock(&cpu_bitmask_lock); ret = __cpu_up(cpu); - mutex_unlock(&cpu_bitmask_lock); if (ret != 0) goto out_notify; BUG_ON(!cpu_online(cpu)); @@ -258,6 +327,7 @@ out_notify: __raw_notifier_call_chain(&cpu_chain, CPU_UP_CANCELED | mod, hcpu, nr_calls, NULL); raw_notifier_call_chain(&cpu_chain, CPU_LOCK_RELEASE, hcpu); + cpu_hotplug_done(); return ret; } @@ -275,13 +345,13 @@ int __cpuinit cpu_up(unsigned int cpu) return -EINVAL; } - mutex_lock(&cpu_add_remove_lock); + cpu_maps_update_begin(); if (cpu_hotplug_disabled) err = -EBUSY; else err = _cpu_up(cpu, 0); - mutex_unlock(&cpu_add_remove_lock); + cpu_maps_update_done(); return err; } @@ -292,7 +362,7 @@ int disable_nonboot_cpus(void) { int cpu, first_cpu, error = 0; - mutex_lock(&cpu_add_remove_lock); + cpu_maps_update_begin(); first_cpu = first_cpu(cpu_online_map); /* We take down all of the non-boot CPUs in one shot to avoid races * with the userspace trying to use the CPU hotplug at the same time @@ -319,7 +389,7 @@ int disable_nonboot_cpus(void) } else { printk(KERN_ERR "Non-boot CPUs are not disabled\n"); } - mutex_unlock(&cpu_add_remove_lock); + cpu_maps_update_done(); return error; } @@ -328,7 +398,7 @@ void enable_nonboot_cpus(void) int cpu, error; /* Allow everyone to use the CPU hotplug again */ - mutex_lock(&cpu_add_remove_lock); + cpu_maps_update_begin(); cpu_hotplug_disabled = 0; if (cpus_empty(frozen_cpus)) goto out; @@ -344,6 +414,6 @@ void enable_nonboot_cpus(void) } cpus_clear(frozen_cpus); out: - mutex_unlock(&cpu_add_remove_lock); + cpu_maps_update_done(); } #endif /* CONFIG_PM_SLEEP_SMP */ -- 2.20.1