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

[Vincent Guittot]

On Tue, 9 Nov 2021 at 19:47, Michal Koutný <mkoutny@xxxxxxxx> wrote:
>
> 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)

As we are not unlinked from tg tree, parent can walk_tg_tree_from and
add it back

>                   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);
>
>