[Devel] Re: [PATCH v3 12/13] execute the whole memcg freeing in rcu callback

[Michal Hocko]

On Tue 18-09-12 18:04:09, Glauber Costa wrote:
> A lot of the initialization we do in mem_cgroup_create() is done with softirqs
> enabled. This include grabbing a css id, which holds &ss->id_lock->rlock, and
> the per-zone trees, which holds rtpz->lock->rlock. All of those signal to the
> lockdep mechanism that those locks can be used in SOFTIRQ-ON-W context. This
> means that the freeing of memcg structure must happen in a compatible context,
> otherwise we'll get a deadlock.

Maybe I am missing something obvious but why cannot we simply disble
(soft)irqs in mem_cgroup_create rather than make the free path much more
complicated. It really feels strange to defer everything (e.g. soft
reclaim tree cleanup which should be a no-op at the time because there
shouldn't be any user pages in the group).

> The reference counting mechanism we use allows the memcg structure to be freed
> later and outlive the actual memcg destruction from the filesystem. However, we
> have little, if any, means to guarantee in which context the last memcg_put
> will happen. The best we can do is test it and try to make sure no invalid
> context releases are happening. But as we add more code to memcg, the possible
> interactions grow in number and expose more ways to get context conflicts.
> 
> We already moved a part of the freeing to a worker thread to be context-safe
> for the static branches disabling. I see no reason not to do it for the whole
> freeing action. I consider this to be the safe choice.


> 
> Signed-off-by: Glauber Costa <glommer at parallels.com>
> Tested-by: Greg Thelen <gthelen at google.com>
> CC: KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
> CC: Michal Hocko <mhocko at suse.cz>
> CC: Johannes Weiner <hannes at cmpxchg.org>
> ---
>  mm/memcontrol.c | 66 +++++++++++++++++++++++++++++----------------------------
>  1 file changed, 34 insertions(+), 32 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index b05ecac..74654f0 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5082,16 +5082,29 @@ out_free:
>  }
>  
>  /*
> - * Helpers for freeing a kmalloc()ed/vzalloc()ed mem_cgroup by RCU,
> - * but in process context.  The work_freeing structure is overlaid
> - * on the rcu_freeing structure, which itself is overlaid on memsw.
> + * At destroying mem_cgroup, references from swap_cgroup can remain.
> + * (scanning all at force_empty is too costly...)
> + *
> + * Instead of clearing all references at force_empty, we remember
> + * the number of reference from swap_cgroup and free mem_cgroup when
> + * it goes down to 0.
> + *
> + * Removal of cgroup itself succeeds regardless of refs from swap.
>   */
> -static void free_work(struct work_struct *work)
> +
> +static void __mem_cgroup_free(struct mem_cgroup *memcg)
>  {
> -	struct mem_cgroup *memcg;
> +	int node;
>  	int size = sizeof(struct mem_cgroup);
>  
> -	memcg = container_of(work, struct mem_cgroup, work_freeing);
> +	mem_cgroup_remove_from_trees(memcg);
> +	free_css_id(&mem_cgroup_subsys, &memcg->css);
> +
> +	for_each_node(node)
> +		free_mem_cgroup_per_zone_info(memcg, node);
> +
> +	free_percpu(memcg->stat);
> +
>  	/*
>  	 * We need to make sure that (at least for now), the jump label
>  	 * destruction code runs outside of the cgroup lock. This is because
> @@ -5110,38 +5123,27 @@ static void free_work(struct work_struct *work)
>  		vfree(memcg);
>  }
>  
> -static void free_rcu(struct rcu_head *rcu_head)
> -{
> -	struct mem_cgroup *memcg;
> -
> -	memcg = container_of(rcu_head, struct mem_cgroup, rcu_freeing);
> -	INIT_WORK(&memcg->work_freeing, free_work);
> -	schedule_work(&memcg->work_freeing);
> -}
>  
>  /*
> - * At destroying mem_cgroup, references from swap_cgroup can remain.
> - * (scanning all at force_empty is too costly...)
> - *
> - * Instead of clearing all references at force_empty, we remember
> - * the number of reference from swap_cgroup and free mem_cgroup when
> - * it goes down to 0.
> - *
> - * Removal of cgroup itself succeeds regardless of refs from swap.
> + * Helpers for freeing a kmalloc()ed/vzalloc()ed mem_cgroup by RCU,
> + * but in process context.  The work_freeing structure is overlaid
> + * on the rcu_freeing structure, which itself is overlaid on memsw.
>   */
> -
> -static void __mem_cgroup_free(struct mem_cgroup *memcg)
> +static void free_work(struct work_struct *work)
>  {
> -	int node;
> +	struct mem_cgroup *memcg;
>  
> -	mem_cgroup_remove_from_trees(memcg);
> -	free_css_id(&mem_cgroup_subsys, &memcg->css);
> +	memcg = container_of(work, struct mem_cgroup, work_freeing);
> +	__mem_cgroup_free(memcg);
> +}
>  
> -	for_each_node(node)
> -		free_mem_cgroup_per_zone_info(memcg, node);
> +static void free_rcu(struct rcu_head *rcu_head)
> +{
> +	struct mem_cgroup *memcg;
>  
> -	free_percpu(memcg->stat);
> -	call_rcu(&memcg->rcu_freeing, free_rcu);
> +	memcg = container_of(rcu_head, struct mem_cgroup, rcu_freeing);
> +	INIT_WORK(&memcg->work_freeing, free_work);
> +	schedule_work(&memcg->work_freeing);
>  }
>  
>  static void mem_cgroup_get(struct mem_cgroup *memcg)
> @@ -5153,7 +5155,7 @@ static void __mem_cgroup_put(struct mem_cgroup *memcg, int count)
>  {
>  	if (atomic_sub_and_test(count, &memcg->refcnt)) {
>  		struct mem_cgroup *parent = parent_mem_cgroup(memcg);
> -		__mem_cgroup_free(memcg);
> +		call_rcu(&memcg->rcu_freeing, free_rcu);
>  		if (parent)
>  			mem_cgroup_put(parent);
>  	}
> -- 
> 1.7.11.4
> 
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-- 
Michal Hocko
SUSE Labs