Re: speed regression in udp_lib_lport_inuse()

[Vitaly Mayatskikh <v.mayatskih@gmail.com>]

At Fri, 23 Jan 2009 14:44:22 +0100, Eric Dumazet wrote:

> Here is an updated of patch, that still have an uniform random distribution,
> keeping a starting point in the range [low - high].
> 
> My attempt to avoid a divide failed ;)
> 
> 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@gmail.com>
> Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
> ---
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index cf5ab05..6e27868 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 +135,16 @@ 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 +167,42 @@ 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 = rand % remaining + low;
> +		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;
> +			/*
> +			 * PORTS_PER_CHAIN loops, because snum is unsigned short
> +			 * and we add an odd multiple of UDP_HTABLE_SIZE
> +			 */
>  			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)) {
> 

Both randomness and speed are good. There're several never bound ports
in the test, but only few. Thank you for the patch.

--
wbr, Vitaly