[PATCH v1] vmpressure: wake up work only when there is registration event

[PATCH v1] vmpressure: wake up work only when there is registration event

[yongw.pur]

From: wangyong <wang.yong12@zte.com.cn>

Use the global variable num_events to record the number of vmpressure
events registered by the system, and wake up work only when there is
registration event.
Usually, the vmpressure event is not registered in the system, this patch
can avoid waking up work and doing nothing.

Refer to Michal Hocko's suggestion:
https://lore.kernel.org/linux-mm/YR%2FNRJEhPKRQ1r22@dhcp22.suse.cz/

Tested-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: wangyong <wang.yong12@zte.com.cn>
---
 mm/vmpressure.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/vmpressure.c b/mm/vmpressure.c
index 76518e4..dfac76b 100644
--- a/mm/vmpressure.c
+++ b/mm/vmpressure.c
@@ -67,6 +67,11 @@ static const unsigned int vmpressure_level_critical = 95;
  */
 static const unsigned int vmpressure_level_critical_prio = ilog2(100 / 10);
 
+/*
+ * Count the number of vmpressure events registered in the system.
+ */
+static atomic_t num_events = ATOMIC_INIT(0);
+
 static struct vmpressure *work_to_vmpressure(struct work_struct *work)
 {
 	return container_of(work, struct vmpressure, work);
@@ -277,7 +282,7 @@ void vmpressure(gfp_t gfp, struct mem_cgroup *memcg, bool tree,
 		vmpr->tree_reclaimed += reclaimed;
 		spin_unlock(&vmpr->sr_lock);
 
-		if (scanned < vmpressure_win)
+		if (scanned < vmpressure_win || atomic_read(&num_events) == 0)
 			return;
 		schedule_work(&vmpr->work);
 	} else {
@@ -407,6 +412,7 @@ int vmpressure_register_event(struct mem_cgroup *memcg,
 	mutex_lock(&vmpr->events_lock);
 	list_add(&ev->node, &vmpr->events);
 	mutex_unlock(&vmpr->events_lock);
+	atomic_add(1, &num_events);
 	ret = 0;
 out:
 	kfree(spec_orig);
@@ -435,6 +441,7 @@ void vmpressure_unregister_event(struct mem_cgroup *memcg,
 		if (ev->efd != eventfd)
 			continue;
 		list_del(&ev->node);
+		atomic_sub(1, &num_events);
 		kfree(ev);
 		break;
 	}
-- 
2.7.4

Re: [PATCH v1] vmpressure: wake up work only when there is registration event

[Michal Hocko]

On Mon 13-09-21 08:54:01, yongw.pur@gmail.com wrote:
> From: wangyong <wang.yong12@zte.com.cn>
> 
> Use the global variable num_events to record the number of vmpressure
> events registered by the system, and wake up work only when there is
> registration event.
> Usually, the vmpressure event is not registered in the system, this patch
> can avoid waking up work and doing nothing.

How much of an improvement does this bring?

> Refer to Michal Hocko's suggestion:
> https://lore.kernel.org/linux-mm/YR%2FNRJEhPKRQ1r22@dhcp22.suse.cz/

let me also point out that we do have means to implement conditional
branches with a zero overhead. Have a look at static branches.

> Tested-by: Zeal Robot <zealci@zte.com.cn>
> Signed-off-by: wangyong <wang.yong12@zte.com.cn>
> ---
>  mm/vmpressure.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vmpressure.c b/mm/vmpressure.c
> index 76518e4..dfac76b 100644
> --- a/mm/vmpressure.c
> +++ b/mm/vmpressure.c
> @@ -67,6 +67,11 @@ static const unsigned int vmpressure_level_critical = 95;
>   */
>  static const unsigned int vmpressure_level_critical_prio = ilog2(100 / 10);
>  
> +/*
> + * Count the number of vmpressure events registered in the system.
> + */
> +static atomic_t num_events = ATOMIC_INIT(0);
> +
>  static struct vmpressure *work_to_vmpressure(struct work_struct *work)
>  {
>  	return container_of(work, struct vmpressure, work);
> @@ -277,7 +282,7 @@ void vmpressure(gfp_t gfp, struct mem_cgroup *memcg, bool tree,
>  		vmpr->tree_reclaimed += reclaimed;
>  		spin_unlock(&vmpr->sr_lock);
>  
> -		if (scanned < vmpressure_win)
> +		if (scanned < vmpressure_win || atomic_read(&num_events) == 0)
>  			return;
>  		schedule_work(&vmpr->work);
>  	} else {

This is a very odd place to put the check on. It is past locks being
held and schedule_work on it's own is unlikely to provide a big
overhead. I would have expected to implement the check at the very
beginning of vmpressure()

-- 
Michal Hocko
SUSE Labs