Commit 73a7242a authored by Waiman Long's avatar Waiman Long Committed by Tejun Heo

cgroup: Keep accurate count of tasks in each css_set

The reference count in the css_set data structure was used as a
proxy of the number of tasks attached to that css_set. However, that
count is actually not an accurate measure especially with thread mode
support. So a new variable nr_tasks is added to the css_set to keep
track of the actual task count. This new variable is protected by
the css_set_lock. Functions that require the actual task count are
updated to use the new variable.

tj: s/task_count/nr_tasks/ for consistency with cgroup_root->nr_cgrps.
    Refreshed on top of cgroup/for-v4.13 which dropped on
    css_set_populated() -> nr_tasks conversion.
Signed-off-by: default avatarWaiman Long <>
Signed-off-by: default avatarTejun Heo <>
parent 63f700aa
......@@ -166,6 +166,9 @@ struct css_set {
/* the default cgroup associated with this css_set */
struct cgroup *dfl_cgrp;
/* internal task count, protected by css_set_lock */
int nr_tasks;
* Lists running through all tasks using this cgroup group.
* mg_tasks lists tasks which belong to this cset but are in the
......@@ -334,10 +334,6 @@ static struct cgroup_pidlist *cgroup_pidlist_find_create(struct cgroup *cgrp,
* cgroup_task_count - count the number of tasks in a cgroup.
* @cgrp: the cgroup in question
* Return the number of tasks in the cgroup. The returned number can be
* higher than the actual number of tasks due to css_set references from
* namespace roots and temporary usages.
static int cgroup_task_count(const struct cgroup *cgrp)
......@@ -346,7 +342,7 @@ static int cgroup_task_count(const struct cgroup *cgrp)
list_for_each_entry(link, &cgrp->cset_links, cset_link)
count += refcount_read(&link->cset->refcount);
count += link->cset->nr_tasks;
return count;
......@@ -573,6 +573,11 @@ static int css_set_count = 1; /* 1 for init_css_set */
* css_set_populated - does a css_set contain any tasks?
* @cset: target css_set
* css_set_populated() should be the same as !!cset->nr_tasks at steady
* state. However, css_set_populated() can be called while a task is being
* added to or removed from the linked list before the nr_tasks is
* properly updated. Hence, we can't just look at ->nr_tasks here.
static bool css_set_populated(struct css_set *cset)
......@@ -1598,6 +1603,7 @@ static void cgroup_enable_task_cg_lists(void)
css_set_update_populated(cset, true);
list_add_tail(&p->cg_list, &cset->tasks);
} while_each_thread(g, p);
......@@ -2064,8 +2070,10 @@ static int cgroup_migrate_execute(struct cgroup_mgctx *mgctx)
struct css_set *to_cset = cset->mg_dst_cset;
css_set_move_task(task, from_cset, to_cset, true);
......@@ -4789,6 +4797,7 @@ void cgroup_post_fork(struct task_struct *child)
cset = task_css_set(current);
if (list_empty(&child->cg_list)) {
css_set_move_task(child, NULL, cset, false);
......@@ -4838,6 +4847,7 @@ void cgroup_exit(struct task_struct *tsk)
if (!list_empty(&tsk->cg_list)) {
css_set_move_task(tsk, cset, NULL, false);
} else {
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