Hi Eric,
On Mon, 2016-12-05 at 09:57 -0800, Eric Dumazet wrote:
> From: Eric Dumazet <eduma...@google.com>
>
> 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 <eduma...@google.com>
> ---
> 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
