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authorEric Dumazet <eric.dumazet@gmail.com>2009-06-11 02:55:43 -0700
committerDavid S. Miller <davem@davemloft.net>2009-06-11 02:55:43 -0700
commit2b85a34e911bf483c27cfdd124aeb1605145dc80 (patch)
tree3cea3e8a27b62de2f92e759641c27200d8bde421 /net/ipv4
parentf2333a014c1e13ac8e1b73a6fd77731c524eff78 (diff)
net: No more expensive sock_hold()/sock_put() on each tx
One of the problem with sock memory accounting is it uses a pair of sock_hold()/sock_put() for each transmitted packet. This slows down bidirectional flows because the receive path also needs to take a refcount on socket and might use a different cpu than transmit path or transmit completion path. So these two atomic operations also trigger cache line bounces. We can see this in tx or tx/rx workloads (media gateways for example), where sock_wfree() can be in top five functions in profiles. We use this sock_hold()/sock_put() so that sock freeing is delayed until all tx packets are completed. As we also update sk_wmem_alloc, we could offset sk_wmem_alloc by one unit at init time, until sk_free() is called. Once sk_free() is called, we atomic_dec_and_test(sk_wmem_alloc) to decrement initial offset and atomicaly check if any packets are in flight. skb_set_owner_w() doesnt call sock_hold() anymore sock_wfree() doesnt call sock_put() anymore, but check if sk_wmem_alloc reached 0 to perform the final freeing. Drawback is that a skb->truesize error could lead to unfreeable sockets, or even worse, prematurely calling __sk_free() on a live socket. Nice speedups on SMP. tbench for example, going from 2691 MB/s to 2711 MB/s on my 8 cpu dev machine, even if tbench was not really hitting sk_refcnt contention point. 5 % speedup on a UDP transmit workload (depends on number of flows), lowering TX completion cpu usage. Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net/ipv4')
-rw-r--r--net/ipv4/ip_output.c1
1 files changed, 0 insertions, 1 deletions
diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 9248d2807ba..24702628266 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -498,7 +498,6 @@ int ip_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
BUG_ON(frag->sk);
if (skb->sk) {
- sock_hold(skb->sk);
frag->sk = skb->sk;
frag->destructor = sock_wfree;
truesizes += frag->truesize;