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

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 <longman@redhat.com>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent 63f700aa

include/linux/cgroup-defs.h

+3 −0
Original line number Diff line number Diff line
@@ -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

kernel/cgroup/cgroup-v1.c

+1 −5
Original line number Diff line number Diff line
@@ -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;

}

kernel/cgroup/cgroup.c

+10 −0
Original line number Diff line number Diff line
@@ -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);

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