Subject: Re: [PATCH] memcg/tcp: fix warning caused b res->usage go to negative.

From: KAMEZAWA Hiroyuki <[email-masked]>
Date: Wed, 18 Apr 2012 17:02:54 +0900

(2012/04/14 2:33), Glauber Costa wrote:

> On 04/09/2012 11:37 PM, KAMEZAWA Hiroyuki wrote:
>> (2012/04/07 0:49), Glauber Costa wrote:
>>
>>> On 03/30/2012 05:44 AM, KAMEZAWA Hiroyuki wrote:
>>>> Maybe what we can do before lsf/mm summit will be this (avoid warning.)
>>>> This patch is onto linus's git tree. Patch description is updated.
>>>>
>>>> Thanks.
>>>> -Kame
>>>> ==
>>>>   From 4ab80f84bbcb02a790342426c1de84aeb17fcbe9 Mon Sep 17 00:00:00 2001
>>>> From: KAMEZAWA Hiroyuki<[email-masked]>
>>>> Date: Thu, 29 Mar 2012 14:59:04 +0900
>>>> Subject: [PATCH] memcg/tcp: fix warning caused b res->usage go to negative.
>>>>
>>>> tcp memcontrol starts accouting after res->limit is set. So, if a sockets
>>>> starts before setting res->limit, there are already used resource.
>>>> At setting res->limit, accounting starts. The resource will be uncharged
>>>> and make res_counter below 0 because they are not charged.
>>>> This causes warning.
>>>>
>>>
>>> Kame,
>>>
>>> Please test the following patch and see if it fixes your problems (I
>>> tested locally, and it triggers me no warnings running the test script
>>> you provided + an inbound scp -r copy of an iso directory from a remote
>>> machine)
>>>
>>> When you are reviewing, keep in mind that we're likely to have the same
>>> problems with slab jump labels - since the slab pages will outlive the
>>> cgroup as well, and it might be worthy to keep this in mind, and provide
>>> a central point for the jump labels to be set of on cgroup destruction.
>>>
>>
>>
>> Hm. What happens in following sequence ?
>>
>>    1. a memcg is created
>>    2. put a task into the memcg, start tcp steam
>>    3. set tcp memory limit
>>
>> The resource used between 2 and 3 will cause the problem finally.
>>
>> Then, Dave's request
>> ==
>> You must either:
>>
>> 1) Integrate the socket's existing usage when the limit is set.
>>
>> 2) Avoid accounting completely for a socket that started before
>>     the limit was set.
>> ==
>> are not satisfied. So, we need to have a state per sockets, it's accounted
>> or not. I'll look into this problem again, today.
>>
> 
> Kame,
> 
> Let me know what you think of the attached fix.
> I still need to compile test it in other configs to be sure it doesn't
> break, etc. But let's agree on it first.

> Subject: [PATCH] decrement static keys on real destroy time
> 
> 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        |    1 +
>  mm/memcontrol.c           |   20 ++++++++++++++++++--
>  net/ipv4/tcp_memcontrol.c |   12 ++++++------
>  3 files changed, 25 insertions(+), 8 deletions(-)
> 
> diff --git a/include/net/sock.h b/include/net/sock.h
> index b3ebe6b..f35ff7d 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -913,6 +913,7 @@ struct cg_proto {
>  	struct percpu_counter	*sockets_allocated;	/* Current number of sockets. */
>  	int			*memory_pressure;
>  	long			*sysctl_mem;
> +	bool			limited;

please add comment somewhere. Hmm, 'limited' is good name ?

>  	/*
>  	 * memcg field is used to find which memcg we belong directly
>  	 * Each memcg struct can hold more than one cg_proto, so container_of
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 02b01d2..61f2d31 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->limited) {
>  			mem_cgroup_get(memcg);
> -			sk->sk_cgrp = sk->sk_prot->proto_cgroup(memcg);
> +			sk->sk_cgrp = cg_proto;
>  		}

Ok, then, sk->sk_cgroup is set only after jump_label is enabled and
its memcg has limitation.

>  		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.limited)
> +		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..8005667 100644
> --- a/net/ipv4/tcp_memcontrol.c
> +++ b/net/ipv4/tcp_memcontrol.c
> @@ -54,6 +54,7 @@ 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->limited = false;
>  	cg_proto->memcg = memcg;
>  
>  	return 0;
> @@ -74,9 +75,6 @@ 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);
>  
> @@ -107,10 +105,12 @@ 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)
> +	if (val == RESOURCE_MAX)
> +		cg_proto->limited = false;

Why we can reset this to be false ? disarm_static_keys() will not work
at destroy()....

> +	else if (val != RESOURCE_MAX && !cg_proto->limited) {
>  		static_key_slow_inc(&memcg_socket_limit_enabled);
> +		cg_proto->limited = true;
> +	}
>  

Hmm. don't we need any mutex ?

Thanks,
-Kame