Hi Martin,
I reconsider these steps which we do here now and saw some new
improvements/issues.
On Thu, Oct 09, 2014 at 08:46:34AM +0100, Martin Townsend wrote:
> Currently there are potentially 2 skb_copy_expand calls in IPHC
> decompression. This patch replaces this with one call to
> pskb_expand_head. It also checks to see if there is enough headroom
> first to ensure it's only done if necessary.
> As pskb_expand_head must only have one reference the calling code
> now ensures this.
>
> Signed-off-by: Martin Townsend <martin.townsend@xxxxxxxxxx>
> ---
> net/6lowpan/iphc.c | 51 ++++++++++++++++++++++++-------------------------
> net/bluetooth/6lowpan.c | 7 +++++++
> 2 files changed, 32 insertions(+), 26 deletions(-)
>
> diff --git a/net/6lowpan/iphc.c b/net/6lowpan/iphc.c
> index 142eef5..853b4b8 100644
> --- a/net/6lowpan/iphc.c
> +++ b/net/6lowpan/iphc.c
> @@ -174,30 +174,22 @@ static int uncompress_context_based_src_addr(struct sk_buff *skb,
> static int skb_deliver(struct sk_buff *skb, struct ipv6hdr *hdr,
> struct net_device *dev, skb_delivery_cb deliver_skb)
> {
> - struct sk_buff *new;
> int stat;
>
> - new = skb_copy_expand(skb, sizeof(struct ipv6hdr), skb_tailroom(skb),
> - GFP_ATOMIC);
> - kfree_skb(skb);
> -
> - if (!new)
> - return -ENOMEM;
> -
> - skb_push(new, sizeof(struct ipv6hdr));
> - skb_reset_network_header(new);
> - skb_copy_to_linear_data(new, hdr, sizeof(struct ipv6hdr));
> + skb_push(skb, sizeof(struct ipv6hdr));
> + skb_reset_network_header(skb);
> + skb_copy_to_linear_data(skb, hdr, sizeof(struct ipv6hdr));
>
> - new->protocol = htons(ETH_P_IPV6);
> - new->pkt_type = PACKET_HOST;
> - new->dev = dev;
> + skb->protocol = htons(ETH_P_IPV6);
> + skb->pkt_type = PACKET_HOST;
> + skb->dev = dev;
>
> raw_dump_table(__func__, "raw skb data dump before receiving",
> - new->data, new->len);
> + skb->data, skb->len);
>
> - stat = deliver_skb(new, dev);
> + stat = deliver_skb(skb, dev);
>
> - kfree_skb(new);
> + consume_skb(skb);
>
> return stat;
> }
> @@ -460,7 +452,7 @@ int lowpan_process_data(struct sk_buff *skb, struct net_device *dev,
> /* UDP data uncompression */
> if (iphc0 & LOWPAN_IPHC_NH_C) {
> struct udphdr uh;
> - struct sk_buff *new;
> + const int needed = sizeof(struct udphdr) + sizeof(hdr);
>
> if (uncompress_udp_header(skb, &uh))
> goto drop;
> @@ -468,14 +460,13 @@ int lowpan_process_data(struct sk_buff *skb, struct net_device *dev,
> /* replace the compressed UDP head by the uncompressed UDP
> * header
> */
> - new = skb_copy_expand(skb, sizeof(struct udphdr),
> - skb_tailroom(skb), GFP_ATOMIC);
> - kfree_skb(skb);
> -
> - if (!new)
> - return -ENOMEM;
> -
> - skb = new;
> + if (skb_headroom(skb) < needed) {
> + err = pskb_expand_head(skb, needed, 0, GFP_ATOMIC);
> + if (unlikely(err)) {
> + kfree_skb(skb);
> + return err;
> + }
> + }
>
> skb_push(skb, sizeof(struct udphdr));
> skb_reset_transport_header(skb);
> @@ -485,6 +476,14 @@ int lowpan_process_data(struct sk_buff *skb, struct net_device *dev,
> (u8 *)&uh, sizeof(uh));
>
> hdr.nexthdr = UIP_PROTO_UDP;
> + } else {
> + if (skb_headroom(skb) < sizeof(hdr)) {
> + err = pskb_expand_head(skb, sizeof(hdr), 0, GFP_ATOMIC);
> + if (unlikely(err)) {
> + kfree_skb(skb);
> + return err;
> + }
> + }
What we here try to do is a usual sk_buff principle.
There exist a function skb_cow [0]:
<qoute>
static inline int __skb_cow(struct sk_buff *skb, unsigned int headroom,
int cloned)
{
int delta = 0;
if (headroom > skb_headroom(skb))
delta = headroom - skb_headroom(skb);
if (delta || cloned)
return pskb_expand_head(skb, ALIGN(delta, NET_SKB_PAD), 0,
GFP_ATOMIC);
return 0;
}
</qoute>
This is the wrapper call which is called by skb_cow. I see here much
similarity and also more performance. It calculates a delta size and do
also a NET_SKB_PAD which is a align to the cache_lines and I think they
try do some "false sharing" here. [1]
We should use this function, if you agree here.
There exist also some skb_cow_head function [2].
code commentar:
<qoute>
It should be used when you only need to push on some header and do not
need to modify the data.
</qoute>
I am not sure about if we can use skb_cow_head here, because we modify
the data. I mean we run skb_pull and then skb_cow_head and run skb_push
with different data buffer. (IPv6 header).
Currently I am not sure if this works.
> }
>
> hdr.payload_len = htons(skb->len);
> diff --git a/net/bluetooth/6lowpan.c b/net/bluetooth/6lowpan.c
> index c2e0d14..6643a7c 100644
> --- a/net/bluetooth/6lowpan.c
> +++ b/net/bluetooth/6lowpan.c
> @@ -343,6 +343,13 @@ static int recv_pkt(struct sk_buff *skb, struct net_device *dev,
> kfree_skb(local_skb);
> kfree_skb(skb);
> } else {
> + /* Decompression may use pskb_expand_head so no shared skb's */
> + skb = skb_share_check(skb, GFP_ATOMIC);
> + if (!skb) {
> + dev->stats.rx_dropped++;
> + return NET_RX_DROP;
> + }
> +
> switch (skb->data[0] & 0xe0) {
> case LOWPAN_DISPATCH_IPHC: /* ipv6 datagram */
> local_skb = skb_clone(skb, GFP_ATOMIC);
Now we come to this function.
There exist two global rules here:
1.
If we change only the skb pointers head/tail with e.g. skb_push then we
need a cloned skb only. We don't change the skb data buffer here.
We need this in LOWPAN_DISPATCH_IPV6, here we run only one skb_pull for
one byte, and the FRAGN and FRAG1 dispatches in 802.15.4 6LoWPAN.
skb_share_check do this, we have surely a clone afterwards and the
sk_buff attributes are not changed elsewhere, which means we are
allowed to manipulate the skb attributes.
2.
If we touch the data buffer, then we need a skb_copy. We need this at
LOWPAN_DISPATCH_IPHC because here we replacing the data buffer.
skb_unshare do this for us.
If we can use skb_cow_head like above, I think then we don't need a
skb_unshare here. But I am not 100% sure if we really can use
skb_cow_head here. We don't need skb_unshare then because, skb_cow_head
makes the head writeable only, this should something similar like a
private copy of headroom then.
Summary:
We need always a skb_share_check at begin of evaluate the dispatch, for
the LOWPAN_DISPATCH_IPHC we need a skb_unshare before. That's fine for
now, maybe later we can do some tricks with skb_cow_head.
skb_share_check:
We always have a cloned skb and we can manipulate sk_buff attributes.
skb_unshare:
We have a private copy of data buffer and can replace 6LoWPAN header
with IPv6 header.
Do you agree here?
- Alex
[0] http://lxr.free-electrons.com/source/include/linux/skbuff.h#L2316
[1] http://en.wikipedia.org/wiki/False_sharing
[2] http://lxr.free-electrons.com/source/include/linux/skbuff.h#L2331
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