Re: [PATCH] sched/fair: Prevent dead task groups from regaining cfs_rq's
From: Michal Koutný
Date: Tue Nov 09 2021 - 13:47:51 EST
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@xxxxxxxxxxxxx> 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);