netdev - Re: [PATCH net-next 6/7] packet: rollover huge flows before small flows

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Date:	Wed, 06 May 2015 12:34:24 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Willem de Bruijn <willemb@...gle.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net
Subject: Re: [PATCH net-next 6/7] packet: rollover huge flows before small
 flows

On Wed, 2015-05-06 at 14:27 -0400, Willem de Bruijn wrote:
> From: Willem de Bruijn <willemb@...gle.com>
> 
> Migrate flows from a socket to another socket in the fanout group not
> only when the socket is full. Start migrating huge flows early, to
> divert possible 4-tuple attacks without affecting normal traffic.
> 
> Introduce fanout_flow_is_huge(). This detects huge flows, which are
> defined as taking up more than half the load. It does so cheaply, by
> storing the rxhashes of the N most recent packets. If over half of
> these are the same rxhash as the current packet, then drop it. This
> only protects against 4-tuple attacks. N is chosen to fit all data in
> a single cache line.
> 
> Tested:
>   Ran bench_rollover for 10 sec with 1.5 Mpps of single flow input.
> 
>       lpbb5:/export/hda3/willemb# ./bench_rollover -l 1000 -r -s
>       cpu        rx       rx.k     drop.k   rollover     r.huge   r.failed
>        0    1202599    1202599          0          0          0          0
>        1    1221096    1221096          0          0          0          0
>        2    1202296    1202296          0          0          0          0
>        3    1229998    1229998          0          0          0          0
>        4    1229551    1229551          0          0          0          0
>        5    1221097    1221097          0          0          0          0
>        6    1223496    1223496          0          0          0          0
>        7    1616768    1616768          0    8530027    8530027          0
> 
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> ---
>  net/packet/af_packet.c | 30 +++++++++++++++++++++++++++---
>  net/packet/internal.h  |  4 ++++
>  2 files changed, 31 insertions(+), 3 deletions(-)
> 
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index d0c4c95..4e54b6b 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -1326,6 +1326,24 @@ static int fanout_rr_next(struct packet_fanout *f, unsigned int num)
>  	return x;
>  }
>  
> +static bool fanout_flow_is_huge(struct packet_sock *po, struct sk_buff *skb)
> +{
> +	u32 rxhash;
> +	int i, count = 0;
> +
> +	rxhash = skb_get_hash(skb);
> +	spin_lock(&po->rollover->hist_lock);
> +	for (i = 0; i < ROLLOVER_HLEN; i++)
> +		if (po->rollover->history[i] == rxhash)
> +			count++;
> +
> +	i = po->rollover->hist_idx++ & (ROLLOVER_HLEN - 1);
> +	po->rollover->history[i] = rxhash;
> +	spin_unlock(&po->rollover->hist_lock);
> +
> +	return count > (ROLLOVER_HLEN >> 1);
> +}
> +

I am not a huge fan of this strategy or memory placement, because of the
spinlock that protects something which should be a hint, more than an
ultra precise decision. At the time lock is released, the status might
already be imprecise.

You touch 3 cache lines here, one for rollover->hist_idx++, one for
history[i] = hash, and one for the spinlock.

(And following patch has the atomic_long_inc() for stats)



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