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SubjectRe: [PATCH 3/3] Convert the UDP hash lock to RCU
Eric Dumazet wrote:
> Corey Minyard a écrit :
>> Change the UDP hash lock from an rwlock to RCU.
>>
>> Signed-off-by: Corey Minyard <cminyard@mvista.com>
>> ---
>> include/net/udp.h | 9 +++++----
>> net/ipv4/udp.c | 47
>> +++++++++++++++++++++++++++--------------------
>> net/ipv6/udp.c | 17 +++++++++--------
>> 3 files changed, 41 insertions(+), 32 deletions(-)
>>
>> diff --git a/include/net/udp.h b/include/net/udp.h
>> index addcdc6..35aa104 100644
>> --- a/include/net/udp.h
>> +++ b/include/net/udp.h
>> @@ -51,7 +51,7 @@ struct udp_skb_cb {
>> #define UDP_SKB_CB(__skb) ((struct udp_skb_cb *)((__skb)->cb))
>>
>> extern struct hlist_head udp_hash[UDP_HTABLE_SIZE];
>> -extern rwlock_t udp_hash_lock;
>> +extern spinlock_t udp_hash_wlock;
>>
>>
>> /* Note: this must match 'valbool' in sock_setsockopt */
>> @@ -112,12 +112,13 @@ static inline void udp_lib_hash(struct sock *sk)
>>
>> static inline void udp_lib_unhash(struct sock *sk)
>> {
>> - write_lock_bh(&udp_hash_lock);
>> - if (sk_del_node_init(sk)) {
>> + spin_lock_bh(&udp_hash_wlock);
>> + if (sk_del_node_init_rcu(sk)) {
>> inet_sk(sk)->num = 0;
>> sock_prot_inuse_add(sock_net(sk), sk->sk_prot, -1);
>> }
>> - write_unlock_bh(&udp_hash_lock);
>> + spin_unlock_bh(&udp_hash_wlock);
>> + synchronize_rcu();
>
> UDP central rwlock can hurt performance, because of cache line ping pong,
> so your patch really makes sense.
>
> Me wondering what impact this synchronize_rcu() can have on mono-threaded
> VOIP applications using lot of UDP sockets. What is the maximum delay of
> this function ?
It delays until all currently executing RCU read-side sections have
executed (new ones don't count, just currently executing ones). I'm not
sure what this delay is, but I would expect it to be fairly small. This
function is only called when a socket is closed, too, so it's not a
high-runner. Paul would certainly know better than me.

>
> For "struct file" freeing, we chose call_rcu() instead of
> synchronize_rcu()
I'd prefer that, too, but that would mean adding another member to the
socket structure.

>
> Maybe we could add a generic rcu head to struct sock, and use
> call_rcu() in
> sk_prot_free() for sockets needing RCU (udp after your patch is
> applied, maybe
> tcp on future patches, while I believe previous work on the subject
> concluded
> RCU was not giving good results for short lived http sessions) ?
RCU probably wouldn't be a good choice for short-lived http sessions,
since you will only get a couple of messages that would matter. I'm not
against adding an item to struct sock, but this is not a common thing
and struct sock was already big and ugly.

>
> Or just add SLAB_DESTROY_BY_RCU to slab creation in proto_register()
> for "struct proto udp_prot/udpv6_prot" so that kmem_cache_free() done
> in sk_prot_free() can defer freeing to RCU...
That's an interesting thought; I didn't know that capability was there.
I can look at that. With this, the short-lived TCP sessions might not
matter, though that's a different issue.

-corey
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