Re: [PATCH] sched/fair: Prevent dead task groups from regaining cfs_rq's

["Michal Koutný" <mkoutny@suse.com>]

Hello.

Let me add a little summary (and I'm CCing cgroups ML too).

The cgroup API allows following callbacks (simplified explanation):

- .css_offline           called after killing main reference 
- .css_released          called when css.refcnt hits zero 
- .css_free              called after css_released and RCU GP 

(The hidden catch is that they're not called directly but through workqueues,
see css_free_work_fn() for details. .css_free() is queued after RCU GP though.)

How is this currently used in the cpu controller:

- .css_offline           not used
- .css_released          sched_offline_group / unregister_fair_sched_group
- .css_free              sched_free_group / free_fair_sched_group


On Mon, Nov 08, 2021 at 12:40:19PM +0100, Peter Zijlstra <peterz@infradead.org> wrote:
> Something like so good?

Thanks for putting this together, in short I understand it reshuffles as follows:

.css_offline    not used
.css_released   cpu_cgroup_css_released
                  sched_released_group(tg)
                    // unlinking from tg tree
.css_free       cpu_cgroup_css_free
                  sched_unregister_group(tg)
                    unregister_fair_sched_group(tg)
                      for_each(cpu)
                        remove_entity_load_avg(cfs_rq)
                        list_del_leaf_cfs_rq(cfs_rq)    (1)
                      destroy_cfs_bandwidth(tg)         (2)
                    call_rcu(sched_free_group_rcu)      (3)
                      [rcu] sched_free_group

I see two following issues with this:

The cfs_bandwidth.period_timer is still active between (1) and (2) and can be
fired and tg_unthrottle_up() may add a cfs_rq back to the leaf list (subsequent
GP won't help).
(Admittedly, this window is shorter than the original window between
cpu_cgroup_css_released() and cpu_cgroup_css_released().)

Having another call_rcu() at (3) seems overly complex given the cgroup API
provides a grace period for free (pun intended :-).


Therefore, symbolically, the symbolic ordering should be:

.css_offline    not used
.css_released   cpu_cgroup_css_released
                  sched_unregister_group(tg)
                    unregister_fair_sched_group(tg)
                      destroy_cfs_bandwidth(tg)
                      for_each(cpu)
                        remove_entity_load_avg(cfs_rq)
                        list_del_leaf_cfs_rq(cfs_rq)
                  sched_released_group(tg)
                    // unlinking from tg tree

.css_free       cpu_cgroup_css_free
                  sched_free_group

That is, the destroy_cfs_bandwidth() is called first to make sure the
tg_unthrottle_up() won't undo some of the cleanups and the respective
structures are only freed after RCU grace period.

(Considering Vincent's remark, remove_entity_load_avg() and
list_del_leaf_cfs_rq() should be swapped but I'd keep that for a different
patch.)

To continue the discussion, the suggestion above in a form of diff (I stripped
the commit message for now).

--8<--
diff --git a/kernel/sched/autogroup.c b/kernel/sched/autogroup.c
index 2067080bb235..8629b37d118e 100644
--- a/kernel/sched/autogroup.c
+++ b/kernel/sched/autogroup.c
@@ -31,7 +31,7 @@ static inline void autogroup_destroy(struct kref *kref)
 	ag->tg->rt_se = NULL;
 	ag->tg->rt_rq = NULL;
 #endif
-	sched_offline_group(ag->tg);
+	sched_release_group(ag->tg);
 	sched_destroy_group(ag->tg);
 }
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 523fd602ea90..a5ca3ae6837b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -9771,12 +9771,12 @@ void sched_destroy_group(struct task_group *tg)
 	call_rcu(&tg->rcu, sched_free_group_rcu);
 }
 
-void sched_offline_group(struct task_group *tg)
+void sched_release_group(struct task_group *tg)
 {
 	unsigned long flags;
 
-	/* End participation in shares distribution: */
 	unregister_fair_sched_group(tg);
+	unregister_rt_sched_group(tg);
 
 	spin_lock_irqsave(&task_group_lock, flags);
 	list_del_rcu(&tg->list);
@@ -9896,7 +9896,7 @@ static void cpu_cgroup_css_released(struct cgroup_subsys_state *css)
 {
 	struct task_group *tg = css_tg(css);
 
-	sched_offline_group(tg);
+	sched_release_group(tg);
 }
 
 static void cpu_cgroup_css_free(struct cgroup_subsys_state *css)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 13950beb01a2..6e476f6d9435 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11456,8 +11456,6 @@ void free_fair_sched_group(struct task_group *tg)
 {
 	int i;
 
-	destroy_cfs_bandwidth(tg_cfs_bandwidth(tg));
-
 	for_each_possible_cpu(i) {
 		if (tg->cfs_rq)
 			kfree(tg->cfs_rq[i]);
@@ -11534,6 +11532,8 @@ void unregister_fair_sched_group(struct task_group *tg)
 	struct rq *rq;
 	int cpu;
 
+	destroy_cfs_bandwidth(tg_cfs_bandwidth(tg));
+
 	for_each_possible_cpu(cpu) {
 		if (tg->se[cpu])
 			remove_entity_load_avg(tg->se[cpu]);
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index bb945f8faeca..b48baaba2fc2 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -137,13 +137,17 @@ static inline struct rq *rq_of_rt_se(struct sched_rt_entity *rt_se)
 	return rt_rq->rq;
 }
 
-void free_rt_sched_group(struct task_group *tg)
+void unregister_rt_sched_group(struct task_group *tg)
 {
-	int i;
-
 	if (tg->rt_se)
 		destroy_rt_bandwidth(&tg->rt_bandwidth);
 
+}
+
+void free_rt_sched_group(struct task_group *tg)
+{
+	int i;
+
 	for_each_possible_cpu(i) {
 		if (tg->rt_rq)
 			kfree(tg->rt_rq[i]);
@@ -250,6 +254,8 @@ static inline struct rt_rq *rt_rq_of_se(struct sched_rt_entity *rt_se)
 	return &rq->rt;
 }
 
+void unregister_rt_sched_group(struct task_group *tg) { }
+
 void free_rt_sched_group(struct task_group *tg) { }
 
 int alloc_rt_sched_group(struct task_group *tg, struct task_group *parent)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 7f1612d26c18..0e66749486e7 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -488,6 +488,7 @@ extern void __refill_cfs_bandwidth_runtime(struct cfs_bandwidth *cfs_b);
 extern void start_cfs_bandwidth(struct cfs_bandwidth *cfs_b);
 extern void unthrottle_cfs_rq(struct cfs_rq *cfs_rq);
 
+extern void unregister_rt_sched_group(struct task_group *tg);
 extern void free_rt_sched_group(struct task_group *tg);
 extern int alloc_rt_sched_group(struct task_group *tg, struct task_group *parent);
 extern void init_tg_rt_entry(struct task_group *tg, struct rt_rq *rt_rq,
@@ -503,7 +504,7 @@ extern struct task_group *sched_create_group(struct task_group *parent);
 extern void sched_online_group(struct task_group *tg,
 			       struct task_group *parent);
 extern void sched_destroy_group(struct task_group *tg);
-extern void sched_offline_group(struct task_group *tg);
+extern void sched_release_group(struct task_group *tg);
 
 extern void sched_move_task(struct task_struct *tsk);