netdev - [PATCH] udp: optimize bind(0) if many ports are in use

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Date:	Mon, 26 Jan 2009 09:20:31 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	David Miller <davem@...emloft.net>
CC:	Vitaly Mayatskikh <v.mayatskih@...il.com>, netdev@...r.kernel.org
Subject: [PATCH] udp: optimize bind(0) if many ports are in use

Hello David

Here is last patch about performance regression of UDP bind() to random port.

Change against previous one is the divide, that can be replaced by a multiply,
as already done in other parts of kernel.

ie instead of doing : "first = rand % remaining + low;" we can use :

first = (((u64)rand * remaining) >> 32) + low;

Also some polishing about too long lines and comments before official submission.

Thank you


[PATCH] udp: optimize bind(0) if many ports are in use

commit 9088c5609584684149f3fb5b065aa7f18dcb03ff
(udp: Improve port randomization) introduced a regression for UDP bind() syscall
to null port (getting a random port) in case lot of ports are already in use.

This is because we do about 28000 scans of very long chains (220 sockets per chain),
with many spin_lock_bh()/spin_unlock_bh() calls.

Fix this using a bitmap (64 bytes for current value of UDP_HTABLE_SIZE)
so that we scan chains at most once.

Instead of 250 ms per bind() call, we get after patch a time of 2.9 ms 

Based on a report from Vitaly Mayatskikh

Reported-by: Vitaly Mayatskikh <v.mayatskih@...il.com>
Signed-off-by: Eric Dumazet <dada1@...mosbay.com>
Tested-by: Vitaly Mayatskikh <v.mayatskih@...il.com>
---

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index cf5ab05..b7faffe 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -120,8 +120,11 @@ EXPORT_SYMBOL(sysctl_udp_wmem_min);
 atomic_t udp_memory_allocated;
 EXPORT_SYMBOL(udp_memory_allocated);
 
+#define PORTS_PER_CHAIN (65536 / UDP_HTABLE_SIZE)
+
 static int udp_lib_lport_inuse(struct net *net, __u16 num,
 			       const struct udp_hslot *hslot,
+			       unsigned long *bitmap,
 			       struct sock *sk,
 			       int (*saddr_comp)(const struct sock *sk1,
 						 const struct sock *sk2))
@@ -132,12 +135,17 @@ static int udp_lib_lport_inuse(struct net *net, __u16 num,
 	sk_nulls_for_each(sk2, node, &hslot->head)
 		if (net_eq(sock_net(sk2), net)			&&
 		    sk2 != sk					&&
-		    sk2->sk_hash == num				&&
+		    (bitmap || sk2->sk_hash == num)		&&
 		    (!sk2->sk_reuse || !sk->sk_reuse)		&&
 		    (!sk2->sk_bound_dev_if || !sk->sk_bound_dev_if
 			|| sk2->sk_bound_dev_if == sk->sk_bound_dev_if) &&
-		    (*saddr_comp)(sk, sk2))
-			return 1;
+		    (*saddr_comp)(sk, sk2)) {
+			if (bitmap)
+				__set_bit(sk2->sk_hash / UDP_HTABLE_SIZE,
+					  bitmap);
+			else
+				return 1;
+		}
 	return 0;
 }
 
@@ -160,32 +168,47 @@ int udp_lib_get_port(struct sock *sk, unsigned short snum,
 	if (!snum) {
 		int low, high, remaining;
 		unsigned rand;
-		unsigned short first;
+		unsigned short first, last;
+		DECLARE_BITMAP(bitmap, PORTS_PER_CHAIN);
 
 		inet_get_local_port_range(&low, &high);
 		remaining = (high - low) + 1;
 
 		rand = net_random();
-		snum = first = rand % remaining + low;
-		rand |= 1;
-		for (;;) {
-			hslot = &udptable->hash[udp_hashfn(net, snum)];
+		first = (((u64)rand * remaining) >> 32) + low;
+		/*
+		 * force rand to be an odd multiple of UDP_HTABLE_SIZE
+		 */
+		rand = (rand | 1) * UDP_HTABLE_SIZE;
+		for (last = first + UDP_HTABLE_SIZE; first != last; first++) {
+			hslot = &udptable->hash[udp_hashfn(net, first)];
+			bitmap_zero(bitmap, PORTS_PER_CHAIN);
 			spin_lock_bh(&hslot->lock);
-			if (!udp_lib_lport_inuse(net, snum, hslot, sk, saddr_comp))
-				break;
-			spin_unlock_bh(&hslot->lock);
+			udp_lib_lport_inuse(net, snum, hslot, bitmap, sk,
+					    saddr_comp);
+
+			snum = first;
+			/*
+			 * Iterate on all possible values of snum for this hash.
+			 * Using steps of an odd multiple of UDP_HTABLE_SIZE
+			 * give us randomization and full range coverage.
+			 */
 			do {
-				snum = snum + rand;
-			} while (snum < low || snum > high);
-			if (snum == first)
-				goto fail;
+				if (low <= snum && snum <= high &&
+				    !test_bit(snum / UDP_HTABLE_SIZE, bitmap))
+					goto found;
+				snum += rand;
+			} while (snum != first);
+			spin_unlock_bh(&hslot->lock);
 		}
+		goto fail;
 	} else {
 		hslot = &udptable->hash[udp_hashfn(net, snum)];
 		spin_lock_bh(&hslot->lock);
-		if (udp_lib_lport_inuse(net, snum, hslot, sk, saddr_comp))
+		if (udp_lib_lport_inuse(net, snum, hslot, NULL, sk, saddr_comp))
 			goto fail_unlock;
 	}
+found:
 	inet_sk(sk)->num = snum;
 	sk->sk_hash = snum;
 	if (sk_unhashed(sk)) {
--
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