Home » Mailing lists » Devel » [PATCH v6 0/2] fix static_key disabling problem in memcg
| Re: [PATCH v6 2/2] decrement static keys on real destroy time [message #46482 is a reply to message #46447] |
Tue, 22 May 2012 22:46   |
akpm
Messages: 224
Registered: March 2007
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Senior Member |
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(cc davem)
On Tue, 22 May 2012 14:25:39 +0400
Glauber Costa <glommer@parallels.com> 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
...
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