cgroup: implement cgroup_has_tasks() and unexport cgroup_task_count()

cgroup_task_count() read-locks css_set_lock and walks all tasks to
count them and then returns the result.  The only thing all the users
want is determining whether the cgroup is empty or not.  This patch
implements cgroup_has_tasks() which tests whether cgroup->cset_links
is empty, replaces all cgroup_task_count() usages and unexports it.

Note that the test isn't synchronized.  This is the same as before.
The test has always been racy.

This will help planned css_set locking update.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index e2ffcdc26cb7e84eadf384c3eb6f8a4a448c2a6a..72154fb44fb51ebd907f6f58e84424c51ea8ab24 100644 (file)
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -457,6 +457,12 @@ static inline bool cgroup_sane_behavior(const struct cgroup *cgrp)
        return cgrp->root->flags & CGRP_ROOT_SANE_BEHAVIOR;
 }
 
+/* no synchronization, the result can only be used as a hint */
+static inline bool cgroup_has_tasks(struct cgroup *cgrp)
+{
+       return !list_empty(&cgrp->cset_links);
+}
+
 /* returns ino associated with a cgroup, 0 indicates unmounted root */
 static inline ino_t cgroup_ino(struct cgroup *cgrp)
 {
@@ -516,8 +522,6 @@ int cgroup_rm_cftypes(struct cftype *cfts);
 
 bool cgroup_is_descendant(struct cgroup *cgrp, struct cgroup *ancestor);
 
-int cgroup_task_count(const struct cgroup *cgrp);
-
 /*
  * Control Group taskset, used to pass around set of tasks to cgroup_subsys
  * methods.

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 2469699408bd0c29911bd2c42b242331457e8ba9..ec7746e5ded1cdf4458dc9c053968405cfe6a862 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2391,7 +2391,7 @@ EXPORT_SYMBOL_GPL(cgroup_add_cftypes);
  *
  * Return the number of tasks in the cgroup.
  */
-int cgroup_task_count(const struct cgroup *cgrp)
+static int cgroup_task_count(const struct cgroup *cgrp)
 {
        int count = 0;
        struct cgrp_cset_link *link;

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index e97a6e88d03691342112a042dc809fa83a71404a..ae190b0a196a3dd7ea4ae736338493a89e6d096c 100644 (file)
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -467,7 +467,7 @@ static int validate_change(struct cpuset *cur, struct cpuset *trial)
         * be changed to have empty cpus_allowed or mems_allowed.
         */
        ret = -ENOSPC;
-       if ((cgroup_task_count(cur->css.cgroup) || cur->attach_in_progress)) {
+       if ((cgroup_has_tasks(cur->css.cgroup) || cur->attach_in_progress)) {
                if (!cpumask_empty(cur->cpus_allowed) &&
                    cpumask_empty(trial->cpus_allowed))
                        goto out;

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c1c25494f7ae278e3feab186856eca9fba97fcc2..d9c6ac1532e64a0dd06be9f36859c35551a15ccc 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4958,7 +4958,7 @@ static int mem_cgroup_force_empty(struct mem_cgroup *memcg)
        struct cgroup *cgrp = memcg->css.cgroup;
 
        /* returns EBUSY if there is a task or if we come here twice. */
-       if (cgroup_task_count(cgrp) || !list_empty(&cgrp->children))
+       if (cgroup_has_tasks(cgrp) || !list_empty(&cgrp->children))
                return -EBUSY;
 
        /* we call try-to-free pages for make this cgroup empty */
@@ -5140,7 +5140,7 @@ static int __memcg_activate_kmem(struct mem_cgroup *memcg,
         * of course permitted.
         */
        mutex_lock(&memcg_create_mutex);
-       if (cgroup_task_count(memcg->css.cgroup) || memcg_has_children(memcg))
+       if (cgroup_has_tasks(memcg->css.cgroup) || memcg_has_children(memcg))
                err = -EBUSY;
        mutex_unlock(&memcg_create_mutex);
        if (err)