cpuset_mutex unlock and lock pair is only needed when transferring tasks
out of empty cpuset. Avoid unneeded cpuset_mutex re-lock when !is_empty
to save cpu cycles.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
---
 kernel/cgroup/cpuset.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 601c40da8e03..e136269c152c 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -3521,17 +3521,16 @@ hotplug_update_tasks_legacy(struct cpuset *cs,
 	is_empty = cpumask_empty(cs->cpus_allowed) ||
 		   nodes_empty(cs->mems_allowed);
 
-	mutex_unlock(&cpuset_mutex);
-
 	/*
 	 * Move tasks to the nearest ancestor with execution resources,
 	 * This is full cgroup operation which will also call back into
 	 * cpuset. Should be done outside any lock.
 	 */
-	if (is_empty)
+	if (is_empty) {
+		mutex_unlock(&cpuset_mutex);
 		remove_tasks_in_empty_cpuset(cs);
-
-	mutex_lock(&cpuset_mutex);
+		mutex_lock(&cpuset_mutex);
+	}
 }
 
 static void
-- 
2.33.0

On 7/4/23 08:03, Miaohe Lin wrote:
> cpuset_mutex unlock and lock pair is only needed when transferring tasks
> out of empty cpuset. Avoid unneeded cpuset_mutex re-lock when !is_empty
> to save cpu cycles.
>
> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> ---
>   kernel/cgroup/cpuset.c | 9 ++++-----
>   1 file changed, 4 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 601c40da8e03..e136269c152c 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -3521,17 +3521,16 @@ hotplug_update_tasks_legacy(struct cpuset *cs,
>   	is_empty = cpumask_empty(cs->cpus_allowed) ||
>   		   nodes_empty(cs->mems_allowed);
>   
> -	mutex_unlock(&cpuset_mutex);
> -
>   	/*
>   	 * Move tasks to the nearest ancestor with execution resources,
>   	 * This is full cgroup operation which will also call back into
>   	 * cpuset. Should be done outside any lock.
>   	 */
> -	if (is_empty)
> +	if (is_empty) {
> +		mutex_unlock(&cpuset_mutex);
>   		remove_tasks_in_empty_cpuset(cs);
> -
> -	mutex_lock(&cpuset_mutex);
> +		mutex_lock(&cpuset_mutex);
> +	}
>   }
>   
>   static void
Reviewed-by: Waiman Long <longman@redhat.com>

On Tue, Jul 04, 2023 at 08:03:52PM +0800, Miaohe Lin wrote:
> cpuset_mutex unlock and lock pair is only needed when transferring tasks
> out of empty cpuset. Avoid unneeded cpuset_mutex re-lock when !is_empty
> to save cpu cycles.
> 
> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>

Applied to cgroup/for-6.6.

Thanks.

-- 
tejun