From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric Dumazet Subject: [PATCH 0/2] udp: Convert the UDP hash lock to RCU Date: Tue, 28 Oct 2008 21:37:15 +0100 Message-ID: <490777FB.2070506@cosmosbay.com> References: <48EB5D28.7000503@cosmosbay.com> <20081007160729.60c076c4@speedy> <20081007.135548.56141000.davem@davemloft.net> <48ECBBD8.9060602@cosmosbay.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: shemminger@vyatta.com, benny+usenet@amorsen.dk, minyard@acm.org, netdev@vger.kernel.org, paulmck@linux.vnet.ibm.com, Christoph Lameter , Peter Zijlstra , Evgeniy Polyakov To: David Miller Return-path: Received: from gw1.cosmosbay.com ([86.65.150.130]:46938 "EHLO gw1.cosmosbay.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752330AbYJ1Uq3 (ORCPT ); Tue, 28 Oct 2008 16:46:29 -0400 In-Reply-To: <48ECBBD8.9060602@cosmosbay.com> Sender: netdev-owner@vger.kernel.org List-ID: UDP sockets are hashed in a 128 slots hash table. This hash table is protected by *one* rwlock. This rwlock is readlocked each time an incoming UDP message is handled. This rwlock is writelocked each time a socket must be inserted in hash table (bind time), or deleted from this table (unbind time) This is not scalable on SMP machines : 1) Even in read mode, lock() and unlock() are atomic operations and must dirty a contended cache line, shared by all cpus. 2) A writer might be starved if many readers are 'in flight'. This can happen on a machine with some NIC receiving many UDP messages. User process can be delayed a long time at socket creation/dismantle time. What Corey and I propose is to use RCU to protect this hash table. Goals are : 1) Optimizing handling of incoming Unicast UDP frames, so that no memory writes should happen in the fast path. Using an array of rwlocks (one per slot for example is not an option in this regard) Note: Multicasts and broadcasts still will need to take a lock, because doing a full lockless lookup in this case is difficult. 2) No expensive operations in the socket bind/unhash phases : - No expensive synchronize_rcu() calls. - No added rcu_head in socket structure, increasing memory needs, but more important, forcing us to use call_rcu() calls, that have the bad property of making sockets structure cold. (rcu grace period between socket freeing and its potential reuse make this socket being cold in CPU cache). David did a previous patch using call_rcu() and noticed a 20% impact on TCP connection rates. Quoting Cristopher Lameter : "Right. That results in cacheline cooldown. You'd want to recycle the object as they are cache hot on a per cpu basis. That is screwed up by the delayed regular rcu processing. We have seen multiple regressions due to cacheline cooldown. The only choice in cacheline hot sensitive areas is to deal with the complexity that comes with SLAB_DESTROY_BY_RCU or give up on RCU." - Because udp sockets are allocated from dedicated kmem_cache, use of SLAB_DESTROY_BY_RCU can help here. Theory of operation : --------------------- As the lookup is lockfree (using rcu_read_lock()/rcu_read_unlock()), special attention must be taken by readers and writers. Use of SLAB_DESTROY_BY_RCU is tricky too, because a socket can be freed, reused, inserted in a different chain or in worst case in the same chain while readers could do lookups in the same time. In order to avoid loops, a reader must check each socket found in a chain really belongs to the chain the reader was traversing. If it finds a mismatch, lookup must start again at the begining. This *restart* loop is the reason we had to use rdlock for the multicast case, because we dont want to send same message several times to the same socket. We use RCU only for fast path. Thus, /proc/net/udp still take rdlocks. Work splited on two patches. [PATCH 1/2] udp: introduce struct udp_table and multiple rwlocks Introduction 'struct udp_table and struct udp_hslot', with one rwlock per chain, instead of a global one. Some cleanups were done to ease review of next patch. [PATCH 2/2] udp: RCU handling for Unicast packets. Tests done on a dual quad core machine (8 cpus) with IPV4 only were pretty good, since some microbenches ran ten times faster. Many thanks to all contributors (David Miller, Christoph Lameter, Peter Zijlstra, Stephen Hemminger, Paul E. McKenney, Evgeniy Polyakov) for their review/comments on initial Corey work.