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: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index ec47101..3bc4196 100644 (file)
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -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

diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index 85d7515..e9ea5f2 100644 (file)
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -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)
 
        spin_lock_irq(&css_set_lock);
        list_for_each_entry(link, &cgrp->cset_links, cset_link)
-               count += refcount_read(&link->cset->refcount);
+               count += link->cset->nr_tasks;
        spin_unlock_irq(&css_set_lock);
        return count;
 }

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 8d4e85e..dbfd702 100644 (file)
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -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);
                        get_css_set(cset);
+                       cset->nr_tasks++;
                }
                spin_unlock(&p->sighand->siglock);
        } 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;
 
                        get_css_set(to_cset);
+                       to_cset->nr_tasks++;
                        css_set_move_task(task, from_cset, to_cset, true);
                        put_css_set_locked(from_cset);
+                       from_cset->nr_tasks--;
                }
        }
        spin_unlock_irq(&css_set_lock);
@@ -4789,6 +4797,7 @@ void cgroup_post_fork(struct task_struct *child)
                cset = task_css_set(current);
                if (list_empty(&child->cg_list)) {
                        get_css_set(cset);
+                       cset->nr_tasks++;
                        css_set_move_task(child, NULL, cset, false);
                }
                spin_unlock_irq(&css_set_lock);
@@ -4838,6 +4847,7 @@ void cgroup_exit(struct task_struct *tsk)
        if (!list_empty(&tsk->cg_list)) {
                spin_lock_irq(&css_set_lock);
                css_set_move_task(tsk, cset, NULL, false);
+               cset->nr_tasks--;
                spin_unlock_irq(&css_set_lock);
        } else {
                get_css_set(cset);