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cgroup: Keep accurate count of tasks in each css_set

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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>
This commit is contained in:
Waiman Long
2017-06-13 17:18:01 -04:00
committed by Tejun Heo
parent 63f700aab4
commit 73a7242a06
3 changed files with 14 additions and 5 deletions
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10
kernel/cgroup/cgroup.c
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@@ -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);