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Subject[RFC PATCH 08/14] cgroup: Keep accurate count of tasks in each css_set
Date
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 task_count 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.

Signed-off-by: Waiman Long <longman@redhat.com>
---
include/linux/cgroup-defs.h | 3 +++
kernel/cgroup/cgroup-v1.c | 6 +-----
kernel/cgroup/cgroup.c | 5 +++++
kernel/cgroup/debug.c | 6 +-----
4 files changed, 10 insertions(+), 10 deletions(-)

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index bb4752a..7be1a90 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -158,6 +158,9 @@ struct css_set {
/* reference count */
atomic_t refcount;

+ /* internal task count, protected by css_set_lock */
+ int task_count;
+
/*
* If not threaded, the following points to self. If threaded, to
* a cset which belongs to the top cgroup of the threaded subtree.
diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index 6757a50..6d69796 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 += atomic_read(&link->cset->refcount);
+ count += link->cset->task_count;
spin_unlock_irq(&css_set_lock);
return count;
}
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index d48eedd..3186b1f 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -1671,6 +1671,7 @@ 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->task_count++;
}
spin_unlock(&p->sighand->siglock);
} while_each_thread(g, p);
@@ -2154,8 +2155,10 @@ 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->task_count++;
css_set_move_task(task, from_cset, to_cset, true);
put_css_set_locked(from_cset);
+ from_cset->task_count--;
}
}
spin_unlock_irq(&css_set_lock);
@@ -5150,6 +5153,7 @@ void cgroup_post_fork(struct task_struct *child)
cset = task_css_set(current);
if (list_empty(&child->cg_list)) {
get_css_set(cset);
+ cset->task_count++;
css_set_move_task(child, NULL, cset, false);
}
spin_unlock_irq(&css_set_lock);
@@ -5199,6 +5203,7 @@ 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->task_count--;
spin_unlock_irq(&css_set_lock);
} else {
get_css_set(cset);
diff --git a/kernel/cgroup/debug.c b/kernel/cgroup/debug.c
index 9146461..c8f7590 100644
--- a/kernel/cgroup/debug.c
+++ b/kernel/cgroup/debug.c
@@ -23,10 +23,6 @@ static void debug_css_free(struct cgroup_subsys_state *css)
/*
* debug_taskcount_read - return 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 u64 debug_taskcount_read(struct cgroup_subsys_state *css,
struct cftype *cft)
@@ -37,7 +33,7 @@ static u64 debug_taskcount_read(struct cgroup_subsys_state *css,

spin_lock_irq(&css_set_lock);
list_for_each_entry(link, &cgrp->cset_links, cset_link)
- count += atomic_read(&link->cset->refcount);
+ count += link->cset->task_count;
spin_unlock_irq(&css_set_lock);
return count;
}
--
1.8.3.1
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