Re: [PATCH v6 30/33] mm: memcontrol: prepare for reparenting non-hierarchical stats

[Michal Koutný]

Hello Qi.

On Thu, Mar 05, 2026 at 07:52:48PM +0800, Qi Zheng <qi.zheng@xxxxxxxxx> wrote:
> To ensure that these non-hierarchical stats work properly, we need to
> reparent these non-hierarchical stats after reparenting LRU folios. To
> this end, this commit makes the following preparations:
> 
> 1. implement reparent_state_local() to reparent non-hierarchical stats
> 2. make css_killed_work_fn() to be called in rcu work, and implement
>    get_non_dying_memcg_start() and get_non_dying_memcg_end() to avoid race
>    between mod_memcg_state()/mod_memcg_lruvec_state()
>    and reparent_state_local()


css_free_rwork_fn has() already RCU deferal but we discussed some
reasons why stats reparenting cannot be done from there. IIUC something
like:

| reparent_state_local() must be already at css_killed_work_fn() because
| waiting until css_free_rwork_fn() would mean the non-hierarchical
| stats of the surrogate ancestor are outdated, e.g. underflown.
| And the waiting until css_free_rwork_fn() may still be indefinite due
| to non-folio references to the offlined memcg.

Could this be captured in the commit (if it's correct)?


> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -6044,8 +6044,9 @@ int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name, umode_t mode)
>   */
>  static void css_killed_work_fn(struct work_struct *work)
>  {
> -	struct cgroup_subsys_state *css =
> -		container_of(work, struct cgroup_subsys_state, destroy_work);
> +	struct cgroup_subsys_state *css;
> +
> +	css = container_of(to_rcu_work(work), struct cgroup_subsys_state, destroy_rwork);
>  
>  	cgroup_lock();
>  
> @@ -6066,8 +6067,8 @@ static void css_killed_ref_fn(struct percpu_ref *ref)
>  		container_of(ref, struct cgroup_subsys_state, refcnt);
>  
>  	if (atomic_dec_and_test(&css->online_cnt)) {
> -		INIT_WORK(&css->destroy_work, css_killed_work_fn);
> -		queue_work(cgroup_offline_wq, &css->destroy_work);
> +		INIT_RCU_WORK(&css->destroy_rwork, css_killed_work_fn);
> +		queue_rcu_work(cgroup_offline_wq, &css->destroy_rwork);
>  	}
>  }
>

Could this be

#ifdef CONFIG_MEMCG_V1
		/* See get_non_dying_memcg_start, get_non_dying_memcg_end
		INIT_RCU_WORK(&css->destroy_rwork, css_killed_work_fn);
		queue_rcu_work(cgroup_offline_wq, &css->destroy_rwork);
#else
		INIT_WORK(&css->destroy_work, css_killed_work_fn);
		queue_work(cgroup_offline_wq, &css->destroy_work);
#endif

?

IOW there's no need to make the cgroup release path even more
asynchronous without CONFIG_MEMCG_V1 (all this seems CONFIG_MEMCG_V1
specific).

(+not so beautiful css_killed_work_fn ifdefing but given there are the
variants of _start, _end)

> +#ifdef CONFIG_MEMCG_V1
> +/*
> + * Used in mod_memcg_state() and mod_memcg_lruvec_state() to avoid race with
> + * reparenting of non-hierarchical state_locals.
> + */
> +static inline struct mem_cgroup *get_non_dying_memcg_start(struct mem_cgroup *memcg)

Nit: I think the idiomatic names are begin + end (in the meaning of
paired parenthesis, don't look at css_task_iter_start ;-).

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