[PATCH] simplify cpu_hotplug_begin()/put_online_cpus()

[PATCH] simplify cpu_hotplug_begin()/put_online_cpus()

[Oleg Nesterov]

cpu_hotplug_begin() must be always called under cpu_add_remove_lock, this means
that only one process can be cpu_hotplug.active_writer. So we don't need the
cpu_hotplug.writer_queue, we can wake up the ->active_writer directly.

Also, fix the comment.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

--- 25/kernel/cpu.c~1_CPU_HP_LOCK	2008-02-15 16:59:17.000000000 +0300
+++ 25/kernel/cpu.c	2008-02-16 18:36:37.000000000 +0300
@@ -33,17 +33,13 @@ static struct {
 	 * an ongoing cpu hotplug operation.
 	 */
 	int refcount;
-	wait_queue_head_t writer_queue;
 } cpu_hotplug;
 
-#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
@@ -65,11 +61,8 @@ void put_online_cpus(void)
 	if (cpu_hotplug.active_writer == current)
 		return;
 	mutex_lock(&cpu_hotplug.lock);
-	cpu_hotplug.refcount--;
-
-	if (unlikely(writer_exists()) && !cpu_hotplug.refcount)
-		wake_up(&cpu_hotplug.writer_queue);
-
+	if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer))
+		wake_up_process(cpu_hotplug.active_writer);
 	mutex_unlock(&cpu_hotplug.lock);
 
 }
@@ -98,8 +91,8 @@ void cpu_maps_update_done(void)
  * 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.
+ * Since cpu_hotplug_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
@@ -115,19 +108,16 @@ void cpu_maps_update_done(void)
  */
 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);
+
+	for (;;) {
+		mutex_lock(&cpu_hotplug.lock);
+		if (likely(!cpu_hotplug.refcount))
+			break;
+		__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)

Re: [PATCH] simplify cpu_hotplug_begin()/put_online_cpus()

[Gautham R Shenoy]

On Sat, Feb 16, 2008 at 08:22:54PM +0300, Oleg Nesterov wrote:

This looks neat and clean.

Acked-by: Gautham R Shenoy <ego@in.ibm.com>


> cpu_hotplug_begin() must be always called under cpu_add_remove_lock, this means
> that only one process can be cpu_hotplug.active_writer. So we don't need the
> cpu_hotplug.writer_queue, we can wake up the ->active_writer directly.
> 
> Also, fix the comment.
> 
> Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
> 
> --- 25/kernel/cpu.c~1_CPU_HP_LOCK	2008-02-15 16:59:17.000000000 +0300
> +++ 25/kernel/cpu.c	2008-02-16 18:36:37.000000000 +0300
> @@ -33,17 +33,13 @@ static struct {
>  	 * an ongoing cpu hotplug operation.
>  	 */
>  	int refcount;
> -	wait_queue_head_t writer_queue;
>  } cpu_hotplug;
> 
> -#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
> @@ -65,11 +61,8 @@ void put_online_cpus(void)
>  	if (cpu_hotplug.active_writer == current)
>  		return;
>  	mutex_lock(&cpu_hotplug.lock);
> -	cpu_hotplug.refcount--;
> -
> -	if (unlikely(writer_exists()) && !cpu_hotplug.refcount)
> -		wake_up(&cpu_hotplug.writer_queue);
> -
> +	if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer))
> +		wake_up_process(cpu_hotplug.active_writer);
>  	mutex_unlock(&cpu_hotplug.lock);
> 
>  }
> @@ -98,8 +91,8 @@ void cpu_maps_update_done(void)
>   * 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.
> + * Since cpu_hotplug_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
> @@ -115,19 +108,16 @@ void cpu_maps_update_done(void)
>   */
>  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);
> +
> +	for (;;) {
> +		mutex_lock(&cpu_hotplug.lock);
> +		if (likely(!cpu_hotplug.refcount))
> +			break;
> +		__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)

-- 
Thanks and Regards
gautham

Re: [PATCH] simplify cpu_hotplug_begin()/put_online_cpus()

[Ingo Molnar]

* Gautham R Shenoy <ego@in.ibm.com> wrote:

> On Sat, Feb 16, 2008 at 08:22:54PM +0300, Oleg Nesterov wrote:
> 
> This looks neat and clean.
> 
> Acked-by: Gautham R Shenoy <ego@in.ibm.com>

thanks, picked it up into the scheduler queue.

	Ingo