Subject: Re: [PATCH 3/3] decrement static keys on real destroy time

From: KAMEZAWA Hiroyuki <[email-masked]>
Date: Fri, 20 Apr 2012 16:38:49 +0900

(2012/04/20 7:49), Glauber Costa wrote:

> We call the destroy function when a cgroup starts to be removed,
> such as by a rmdir event.
> 
> However, because of our reference counters, some objects are still
> inflight. Right now, we are decrementing the static_keys at destroy()
> time, meaning that if we get rid of the last static_key reference,
> some objects will still have charges, but the code to properly
> uncharge them won't be run.
> 
> This becomes a problem specially if it is ever enabled again, because
> now new charges will be added to the staled charges making keeping
> it pretty much impossible.
> 
> We just need to be careful with the static branch activation:
> since there is no particular preferred order of their activation,
> we need to make sure that we only start using it after all
> call sites are active. This is achieved by having a per-memcg
> flag that is only updated after static_key_slow_inc() returns.
> At this time, we are sure all sites are active.
> 
> This is made per-memcg, not global, for a reason:
> it also has the effect of making socket accounting more
> consistent. The first memcg to be limited will trigger static_key()
> activation, therefore, accounting. But all the others will then be
> accounted no matter what. After this patch, only limited memcgs
> will have its sockets accounted.
> 
> [v2: changed a tcp limited flag for a generic proto limited flag ]
> 
> Signed-off-by: Glauber Costa <[email-masked]>

> ---
>  include/net/sock.h        |    9 +++++++
>  mm/memcontrol.c           |   20 +++++++++++++++-
>  net/ipv4/tcp_memcontrol.c |   52 ++++++++++++++++++++++++++++++++++++++------
>  3 files changed, 72 insertions(+), 9 deletions(-)
> 
> diff --git a/include/net/sock.h b/include/net/sock.h
> index b3ebe6b..c5a2010 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -914,6 +914,15 @@ struct cg_proto {
>  	int			*memory_pressure;
>  	long			*sysctl_mem;
>  	/*
> +	 * active means it is currently active, and new sockets should
> +	 * be assigned to cgroups.
> +	 *
> +	 * activated means it was ever activated, and we need to
> +	 * disarm the static keys on destruction
> +	 */
> +	bool			activated;
> +	bool			active; 
> +	/*
>  	 * memcg field is used to find which memcg we belong directly
>  	 * Each memcg struct can hold more than one cg_proto, so container_of
>  	 * won't really cut.
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 7832b4d..01d25a0 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -404,6 +404,7 @@ void sock_update_memcg(struct sock *sk)
>  {
>  	if (mem_cgroup_sockets_enabled) {
>  		struct mem_cgroup *memcg;
> +		struct cg_proto *cg_proto;
>  
>  		BUG_ON(!sk->sk_prot->proto_cgroup);
>  
> @@ -423,9 +424,10 @@ void sock_update_memcg(struct sock *sk)
>  
>  		rcu_read_lock();
>  		memcg = mem_cgroup_from_task(current);
> -		if (!mem_cgroup_is_root(memcg)) {
> +		cg_proto = sk->sk_prot->proto_cgroup(memcg);
> +		if (!mem_cgroup_is_root(memcg) && cg_proto->active) {

>  			mem_cgroup_get(memcg);
> -			sk->sk_cgrp = sk->sk_prot->proto_cgroup(memcg);
> +			sk->sk_cgrp = cg_proto;
>  		}

Is this correct ? cg_proto->active can be true before all jump_labels are
patched, then we can loose accounting. That will cause underflow of
res_countner.

cg_proto->active should be set after jump_label modification.
Then, things will work, I guess.

Thanks,
-Kame

>  		rcu_read_unlock();
>  	}
> @@ -442,6 +444,14 @@ void sock_release_memcg(struct sock *sk)
>  	}
>  }
>  
> +static void disarm_static_keys(struct mem_cgroup *memcg)
> +{
> +#ifdef CONFIG_INET
> +	if (memcg->tcp_mem.cg_proto.activated)
> +		static_key_slow_dec(&memcg_socket_limit_enabled);
> +#endif
> +}
> +
>  #ifdef CONFIG_INET
>  struct cg_proto *tcp_proto_cgroup(struct mem_cgroup *memcg)
>  {
> @@ -452,6 +462,11 @@ struct cg_proto *tcp_proto_cgroup(struct mem_cgroup *memcg)
>  }
>  EXPORT_SYMBOL(tcp_proto_cgroup);
>  #endif /* CONFIG_INET */
> +#else
> +static inline void disarm_static_keys(struct mem_cgroup *memcg)
> +{
> +}
> +
>  #endif /* CONFIG_CGROUP_MEM_RES_CTLR_KMEM */
>  
>  static void drain_all_stock_async(struct mem_cgroup *memcg);
> @@ -4883,6 +4898,7 @@ 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);
> +		disarm_static_keys(memcg);
>  		__mem_cgroup_free(memcg);
>  		if (parent)
>  			mem_cgroup_put(parent);
> diff --git a/net/ipv4/tcp_memcontrol.c b/net/ipv4/tcp_memcontrol.c
> index 1517037..d02573a 100644
> --- a/net/ipv4/tcp_memcontrol.c
> +++ b/net/ipv4/tcp_memcontrol.c
> @@ -54,6 +54,8 @@ int tcp_init_cgroup(struct mem_cgroup *memcg, struct cgroup_subsys *ss)
>  	cg_proto->sysctl_mem = tcp->tcp_prot_mem;
>  	cg_proto->memory_allocated = &tcp->tcp_memory_allocated;
>  	cg_proto->sockets_allocated = &tcp->tcp_sockets_allocated;
> +	cg_proto->active = false;
> +	cg_proto->activated = false;
>  	cg_proto->memcg = memcg;
>  
>  	return 0;
> @@ -74,12 +76,23 @@ void tcp_destroy_cgroup(struct mem_cgroup *memcg)
>  	percpu_counter_destroy(&tcp->tcp_sockets_allocated);
>  
>  	val = res_counter_read_u64(&tcp->tcp_memory_allocated, RES_LIMIT);
> -
> -	if (val != RESOURCE_MAX)
> -		static_key_slow_dec(&memcg_socket_limit_enabled);
>  }
>  EXPORT_SYMBOL(tcp_destroy_cgroup);
>  
> +/*
> + * This is to prevent two writes arriving at the same time
> + * at kmem.tcp.limit_in_bytes.
> + *
> + * There is a race at the first time we write to this file:
> + *
> + * - cg_proto->activated == false for all writers.
> + * - They all do a static_key_slow_inc().
> + * - When we are finally read to decrement the static_keys,
> + *   we'll do it only once per activated cgroup. So we won't
> + *   be able to disable it.
> + */
> +static DEFINE_MUTEX(tcp_set_limit_mutex);
> +
>  static int tcp_update_limit(struct mem_cgroup *memcg, u64 val)
>  {
>  	struct net *net = current->nsproxy->net_ns;
> @@ -107,10 +120,35 @@ static int tcp_update_limit(struct mem_cgroup *memcg, u64 val)
>  		tcp->tcp_prot_mem[i] = min_t(long, val >> PAGE_SHIFT,
>  					     net->ipv4.sysctl_tcp_mem[i]);
>  
> -	if (val == RESOURCE_MAX && old_lim != RESOURCE_MAX)
> -		static_key_slow_dec(&memcg_socket_limit_enabled);
> -	else if (old_lim == RESOURCE_MAX && val != RESOURCE_MAX)
> -		static_key_slow_inc(&memcg_socket_limit_enabled);
> +	if (val == RESOURCE_MAX)
> +		cg_proto->active = false;
> +	else if (val != RESOURCE_MAX) {
> +		cg_proto->active = true;
> +
> +
> +		/*
> +		 * ->activated needs to be written after the static_key update.
> +		 *  This is what guarantees that the socket activation function
> +		 *  is the last one to run. See sock_update_memcg() for details,
> +		 *  and note that we don't mark any socket as belonging to this
> +		 *  memcg until that flag is up.
> +		 *
> +		 *  We need to do this, because static_keys will span multiple
> +		 *  sites, but we can't control their order. If we mark a socket
> +		 *  as accounted, but the accounting functions are not patched in
> +		 *  yet, we'll lose accounting.
> +		 *
> +		 *  We never race with the readers in sock_update_memcg(), because
> +		 *  when this value change, the code to process it is not patched in
> +		 *  yet.
> +		 */
> +		mutex_lock(&tcp_set_limit_mutex);
> +		if (!cg_proto->activated) {
> +			static_key_slow_inc(&memcg_socket_limit_enabled);
> +			cg_proto->activated = true;
> +		}
> +		mutex_unlock(&tcp_set_limit_mutex);
> +	}
>  
>  	return 0;
>  }