Commit d221938c authored by Gautham R Shenoy's avatar Gautham R Shenoy Committed by Ingo Molnar

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: default avatarGautham R Shenoy <ego@in.ibm.com>
Signed-off-by: Paul Jackson <pj@sgi.com> [For !CONFIG_HOTPLUG_CPU ]
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 6b2d7700
......@@ -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. */
......
......@@ -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();
......
......@@ -15,9 +15,8 @@
#include <linux/stop_machine.h>
#include <linux/mutex.h>
/* 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 */
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