[PATCH 1/7] cgroup: Keep accurate count of tasks in each css_set

Tejun Heo
2017-06-14T20:08:52Z

From: Waiman Long <longman@redhat.com>

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>
---
include/linux/cgroup-defs.h | 3 +++
kernel/cgroup/cgroup-v1.c | 6 +-----
kernel/cgroup/cgroup.c | 10 ++++++++++
3 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index ec47101cb1bf..3bc4196bf217 100644
--- 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 85d75152402d..e9ea5f201fac 100644
--- 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 8d4e85eae42c..dbfd7028b1c6 100644
--- 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);
--
2.13.0