Subject: Re: [PATCH] net/udp: do not touch skb->peeked unless really needed

From: Paolo Abeni <[email-masked]>
Date: Tue, 06 Dec 2016 10:53:09 +0100

Hi Eric,

On Mon, 2016-12-05 at 09:57 -0800, Eric Dumazet wrote:
> From: Eric Dumazet <[email-masked]>
> 
> In UDP recvmsg() path we currently access 3 cache lines from an skb
> while holding receive queue lock, plus another one if packet is
> dequeued, since we need to change skb->next->prev
> 
> 1st cache line (contains ->next/prev pointers, offsets 0x00 and 0x08)
> 2nd cache line (skb->len & skb->peeked, offsets 0x80 and 0x8e)
> 3rd cache line (skb->truesize/users, offsets 0xe0 and 0xe4)
> 
> skb->peeked is only needed to make sure 0-length packets are properly
> handled while MSG_PEEK is operated.
> 
> I had first the intent to remove skb->peeked but the "MSG_PEEK at
> non-zero offset" support added by Sam Kumar makes this not possible.

I'm wondering if peeking with offset is going to complicate the 2 queues
patch, too.

> This patch avoids one cache line miss during the locked section, when
> skb->len and skb->peeked do not have to be read.
> 
> It also avoids the skb_set_peeked() cost for non empty UDP datagrams.
> 
> Signed-off-by: Eric Dumazet <[email-masked]>
> ---
>  net/core/datagram.c |   19 ++++++++++---------
>  1 file changed, 10 insertions(+), 9 deletions(-)
> 
> diff --git a/net/core/datagram.c b/net/core/datagram.c
> index 49816af8586bb832e806972b486588041a99524c..9482037a5c8c64aec79e42c65bd2691bdd9450a3 100644
> --- a/net/core/datagram.c
> +++ b/net/core/datagram.c
> @@ -214,6 +214,7 @@ struct sk_buff *__skb_try_recv_datagram(struct sock *sk, unsigned int flags,
>  	if (error)
>  		goto no_packet;
>  
> +	*peeked = 0;
>  	do {
>  		/* Again only user level code calls this function, so nothing
>  		 * interrupt level will suddenly eat the receive_queue.
> @@ -227,22 +228,22 @@ struct sk_buff *__skb_try_recv_datagram(struct sock *sk, unsigned int flags,
>  		spin_lock_irqsave(&queue->lock, cpu_flags);
>  		skb_queue_walk(queue, skb) {
>  			*last = skb;
> -			*peeked = skb->peeked;
>  			if (flags & MSG_PEEK) {
>  				if (_off >= skb->len && (skb->len || _off ||
>  							 skb->peeked)) {
>  					_off -= skb->len;
>  					continue;
>  				}
> -
> -				skb = skb_set_peeked(skb);
> -				error = PTR_ERR(skb);
> -				if (IS_ERR(skb)) {
> -					spin_unlock_irqrestore(&queue->lock,
> -							       cpu_flags);
> -					goto no_packet;
> +				if (!skb->len) {
> +					skb = skb_set_peeked(skb);
> +					if (IS_ERR(skb)) {
> +						error = PTR_ERR(skb);
> +						spin_unlock_irqrestore(&queue->lock,
> +								       cpu_flags);
> +						goto no_packet;
> +					}
>  				}

I don't understand why we can avoid setting skb->peek if len > 0. I
think that will change the kernel behavior if:
- peek with offset is set
- 3 skbs with len > 0 are enqueued
- the u/s peek (with offset) the second one
- the u/s disable peeking with offset and peeks 2 more skbs.

With the current code in the last step the u/s is going to peek the 1#
and the 3# skbs, after this patch will peek the 1# and the 2#. Am I
missing something ? Probably the new behavior is more correct, but still
is a change. 

I gave this a run in my test bed on top of your udp-related patches I
see additional ~3 improvement in the udp flood scenario, and a bit more
in the un-contended scenario.

Thank you,

Paolo