Subject: Re: [PATCH v6 2/2] decrement static keys on real destroy time

From: Andrew Morton <akpm-de/[email-masked]>
Date: Tue, 22 May 2012 15:46:10 -0700

(cc davem)

On Tue, 22 May 2012 14:25:39 +0400
Glauber Costa <[email-masked]> 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 ]
> [v3: update the current active flag only after the static_key update ]
> [v4: disarm_static_keys() inside free_work ]
> [v5: got rid of tcp_limit_mutex, now in the static_key interface ]
> [v6: changed active and activated to a flags field, as suggested by akpm ]

A few things...

>  include/linux/memcontrol.h |    5 +++++
>  include/net/sock.h         |   11 +++++++++++
>  mm/memcontrol.c            |   29 +++++++++++++++++++++++++++--
>  net/ipv4/tcp_memcontrol.c  |   34 +++++++++++++++++++++++++++-------
>  4 files changed, 70 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index f94efd2..9dc0b86 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -436,6 +436,11 @@ enum {
>  	OVER_LIMIT,
>  };
>  
> +enum sock_flag_bits {
> +	MEMCG_SOCK_ACTIVE,
> +	MEMCG_SOCK_ACTIVATED,
> +};

I don't see why this was defined in memcontrol.h.  It is enumerating
the bits in sock.h's cg_proto.flags, so why not define it in sock.h? 
This is changed in the appended patch.

Also, in the v5 patch these flags were documented, as they should be. 
Version 6 forgot to do this. This is changed in the appended patch.

And version 6 doesn't describe what sock_flag_bits actually does.  It
should. This is changed in the appended patch.

And the name seems inappropriate to me.  Should it not be enum
cg_proto_flag_bits?  Or, probably better, cg_proto_flags?  This I did
*not* change.

>  struct sock;
>  #ifdef CONFIG_CGROUP_MEM_RES_CTLR_KMEM
>  void sock_update_memcg(struct sock *sk);
> diff --git a/include/net/sock.h b/include/net/sock.h
> index b3ebe6b..1742db7 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;
> +	unsigned long		flags;
>  	/*
>  	 * memcg field is used to find which memcg we belong directly
>  	 * Each memcg struct can hold more than one cg_proto, so container_of
> @@ -928,6 +929,16 @@ struct cg_proto {
>  extern int proto_register(struct proto *prot, int alloc_slab);
>  extern void proto_unregister(struct proto *prot);
>  
> +static inline bool memcg_proto_active(struct cg_proto *cg_proto)
> +{
> +	return cg_proto->flags & (1 << MEMCG_SOCK_ACTIVE);
> +}
> +
> +static inline bool memcg_proto_activated(struct cg_proto *cg_proto)
> +{
> +	return cg_proto->flags & (1 << MEMCG_SOCK_ACTIVATED);
> +}

Here, we're open-coding kinda-test_bit().  Why do that?  These flags are
modified with set_bit() and friends, so we should read them with the
matching test_bit()?

Also, these bool-returning functions will return values other than 0
and 1.  That probably works OK and I don't know what the C standards
and implementations do about this.  But it seems unclean and slightly
risky to have a "bool" value of 32!  Converting these functions to use
test_bit() fixes this - test_bit() returns only 0 or 1.

test_bit() is slightly more expensive than the above.  If this is
considered to be an issue then I guess we could continue to use this
approach.  But I do think a code comment is needed, explaining and
justifying the unusual decision to bypass the bitops API.  Also these
functions should tell the truth and return an "int" type.

>  #ifdef SOCK_REFCNT_DEBUG
>  static inline void sk_refcnt_debug_inc(struct sock *sk)
>  {
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 0b4b4c8..22434bf 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) && memcg_proto_active(cg_proto)) {
>  			mem_cgroup_get(memcg);
> -			sk->sk_cgrp = sk->sk_prot->proto_cgroup(memcg);
> +			sk->sk_cgrp = cg_proto;
>  		}
>  		rcu_read_unlock();
>  	}
> @@ -451,9 +453,25 @@ struct cg_proto *tcp_proto_cgroup(struct mem_cgroup *memcg)
>  	return &memcg->tcp_mem.cg_proto;
>  }
>  EXPORT_SYMBOL(tcp_proto_cgroup);
> +
> +static void disarm_sock_keys(struct mem_cgroup *memcg)
> +{
> +	if (!memcg_proto_activated(&memcg->tcp_mem.cg_proto))
> +		return;
> +	static_key_slow_dec(&memcg_socket_limit_enabled);
> +}
> +#else
> +static void disarm_sock_keys(struct mem_cgroup *memcg)
> +{
> +}
>  #endif /* CONFIG_INET */
>  #endif /* CONFIG_CGROUP_MEM_RES_CTLR_KMEM */
>  
> +static void disarm_static_keys(struct mem_cgroup *memcg)
> +{
> +	disarm_sock_keys(memcg);
> +}

Why does this function exist?  Its single caller could call
disarm_sock_keys() directly.

>  static void drain_all_stock_async(struct mem_cgroup *memcg);
>  
>  static struct mem_cgroup_per_zone *
> @@ -4836,6 +4854,13 @@ static void free_work(struct work_struct *work)
>  	int size = sizeof(struct mem_cgroup);
>  
>  	memcg = container_of(work, struct mem_cgroup, work_freeing);
> +	/*
> +	 * We need to make sure that (at least for now), the jump label
> +	 * destruction code runs outside of the cgroup lock.

This is a poor comment - it failed to tell the reader *why* that code
must run outside the cgroup lock.

>							schedule_work()
> +	 * will guarantee this happens. Be careful if you need to move this
> +	 * disarm_static_keys around

It's a bit difficult for the reader to be careful when he isn't told
what the risks are.

> +	 */
> +	disarm_static_keys(memcg);
>  	if (size < PAGE_SIZE)
>  		kfree(memcg);
>  	else
> diff --git a/net/ipv4/tcp_memcontrol.c b/net/ipv4/tcp_memcontrol.c
> index 1517037..3b8fa25 100644
> --- a/net/ipv4/tcp_memcontrol.c
> +++ b/net/ipv4/tcp_memcontrol.c
> @@ -74,9 +74,6 @@ 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 +104,33 @@ 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)
> +		clear_bit(MEMCG_SOCK_ACTIVE, &cg_proto->flags);
> +	else if (val != RESOURCE_MAX) {
> +		/*
> +		 *  The active bit 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.
> +		 *
> +		 *  The activated bit is used to guarantee that no two writers will 
> +		 *  do the update in the same memcg. Without that, we can't properly
> +		 *  shutdown the static key.
> +		 */

This comment needlessly overflows 80 cols and has a pointless and
unconventional double-space indenting.  I already provided a patch
which fixes this and a few other things, but that was ignored when you
did the v6.

> +		if (!test_and_set_bit(MEMCG_SOCK_ACTIVATED, &cg_proto->flags))
> +			static_key_slow_inc(&memcg_socket_limit_enabled);
> +		set_bit(MEMCG_SOCK_ACTIVE, &cg_proto->flags);
> +	}

So here are suggested changes from *some* of the above discussion. 
Please consider, incorporate, retest and send us a v7?

From: Andrew Morton <akpm-de/[email-masked]>
Subject: memcg-decrement-static-keys-at-real-destroy-time-v6-fix

- move enum sock_flag_bits into sock.h

- document enum sock_flag_bits

- convert memcg_proto_active() and memcg_proto_activated() to test_bit()

- redo tcp_update_limit() comment to 80 cols

Cc: Glauber Costa <[email-masked]>
Cc: Johannes Weiner <[email-masked]>
Cc: Kamezawa Hiroyuki <[email-masked]>
Cc: Li Zefan <[email-masked]>
Cc: Michal Hocko <[email-masked]>
Cc: Tejun Heo <[email-masked]>
Signed-off-by: Andrew Morton <akpm-de/[email-masked]>
---

 include/linux/memcontrol.h |    5 -----
 include/net/sock.h         |   15 +++++++++++++--
 net/ipv4/tcp_memcontrol.c  |   30 +++++++++++++++---------------
 3 files changed, 28 insertions(+), 22 deletions(-)

diff -puN include/linux/memcontrol.h~memcg-decrement-static-keys-at-real-destroy-time-v6-fix include/linux/memcontrol.h
--- a/include/linux/memcontrol.h~memcg-decrement-static-keys-at-real-destroy-time-v6-fix
+++ a/include/linux/memcontrol.h
@@ -405,11 +405,6 @@ enum {
 	OVER_LIMIT,
 };
 
-enum sock_flag_bits {
-	MEMCG_SOCK_ACTIVE,
-	MEMCG_SOCK_ACTIVATED,
-};
-
 struct sock;
 #ifdef CONFIG_CGROUP_MEM_RES_CTLR_KMEM
 void sock_update_memcg(struct sock *sk);
diff -puN include/net/sock.h~memcg-decrement-static-keys-at-real-destroy-time-v6-fix include/net/sock.h
--- a/include/net/sock.h~memcg-decrement-static-keys-at-real-destroy-time-v6-fix
+++ a/include/net/sock.h
@@ -46,6 +46,7 @@
 #include <linux/list_nulls.h>
 #include <linux/timer.h>
 #include <linux/cache.h>
+#include <linux/bitops.h>
 #include <linux/lockdep.h>
 #include <linux/netdevice.h>
 #include <linux/skbuff.h>	/* struct sk_buff */
@@ -921,6 +922,16 @@ struct proto {
 #endif
 };
 
+/*
+ * Bits in struct cg_proto.flags
+ */
+enum sock_flag_bits {
+	/* Currently active and new sockets should be assigned to cgroups */
+	MEMCG_SOCK_ACTIVE,
+	/* It was ever activated; we must disarm static keys on destruction */
+	MEMCG_SOCK_ACTIVATED,
+};
+
 struct cg_proto {
 	void			(*enter_memory_pressure)(struct sock *sk);
 	struct res_counter	*memory_allocated;	/* Current allocated memory. */
@@ -945,12 +956,12 @@ extern void proto_unregister(struct prot
 
 static inline bool memcg_proto_active(struct cg_proto *cg_proto)
 {
-	return cg_proto->flags & (1 << MEMCG_SOCK_ACTIVE);
+	return test_bit(MEMCG_SOCK_ACTIVE, &cg_proto->flags);
 }
 
 static inline bool memcg_proto_activated(struct cg_proto *cg_proto)
 {
-	return cg_proto->flags & (1 << MEMCG_SOCK_ACTIVATED);
+	return test_bit(MEMCG_SOCK_ACTIVATED, &cg_proto->flags);
 }
 
 #ifdef SOCK_REFCNT_DEBUG
diff -puN net/ipv4/tcp_memcontrol.c~memcg-decrement-static-keys-at-real-destroy-time-v6-fix net/ipv4/tcp_memcontrol.c
--- a/net/ipv4/tcp_memcontrol.c~memcg-decrement-static-keys-at-real-destroy-time-v6-fix
+++ a/net/ipv4/tcp_memcontrol.c
@@ -108,24 +108,24 @@ static int tcp_update_limit(struct mem_c
 		clear_bit(MEMCG_SOCK_ACTIVE, &cg_proto->flags);
 	else if (val != RESOURCE_MAX) {
 		/*
-		 *  The active bit 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.
+		 * The active bit 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 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.
+		 * 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.
 		 *
-		 *  The activated bit is used to guarantee that no two writers will
-		 *  do the update in the same memcg. Without that, we can't properly
-		 *  shutdown the static key.
+		 * The activated bit is used to guarantee that no two writers
+		 * will do the update in the same memcg. Without that, we can't
+		 * properly shutdown the static key.
 		 */
 		if (!test_and_set_bit(MEMCG_SOCK_ACTIVATED, &cg_proto->flags))
 			static_key_slow_inc(&memcg_socket_limit_enabled);
_