diff mbox series

[v3,2/2] gro: optimise redundant parsing of packets

Message ID 20230313164541.GA17394@debian (mailing list archive)
State Superseded
Delegated to: Netdev Maintainers
Headers show
Series gro: optimise redundant parsing of packets | expand

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netdev/series_format warning Target tree name not specified in the subject
netdev/tree_selection success Guessed tree name to be net-next
netdev/fixes_present success Fixes tag not required for -next series
netdev/header_inline success No static functions without inline keyword in header files
netdev/build_32bit success Errors and warnings before: 38 this patch: 38
netdev/cc_maintainers success CCed 10 of 10 maintainers
netdev/build_clang success Errors and warnings before: 18 this patch: 18
netdev/verify_signedoff success Signed-off-by tag matches author and committer
netdev/deprecated_api success None detected
netdev/check_selftest success No net selftest shell script
netdev/verify_fixes success No Fixes tag
netdev/build_allmodconfig_warn success Errors and warnings before: 38 this patch: 38
netdev/checkpatch warning WARNING: line length of 89 exceeds 80 columns
netdev/kdoc success Errors and warnings before: 0 this patch: 0
netdev/source_inline success Was 0 now: 0

Commit Message

Richard Gobert March 13, 2023, 4:45 p.m. UTC 
Currently the IPv6 extension headers are parsed twice: first in
ipv6_gro_receive, and then again in ipv6_gro_complete.

By using the new ->transport_proto field, and also storing the size of the
network header, we can avoid parsing extension headers a second time in
ipv6_gro_complete (which saves multiple memory dereferences and conditional
checks inside ipv6_exthdrs_len for a varying amount of extension headers in
IPv6 packets).

The implementation had to handle both inner and outer layers in case of
encapsulation (as they can't use the same field). I've applied a similar
optimisation to Ethernet.

Performance tests for TCP stream over IPv6 with a varying amount of
extension headers demonstrate throughput improvement of ~0.7%.

In addition, I fixed a potential future problem:
 - The call to skb_set_inner_network_header at the beginning of
   ipv6_gro_complete calculates inner_network_header based on skb->data by
   calling skb_set_inner_network_header, and setting it to point to the
   beginning of the ip header.
 - If a packet is going to be handled by BIG TCP, the following code block
   is going to shift the packet header, and skb->data is going to be
   changed as well.

When the two flows are combined, inner_network_header will point to the
wrong place - which might happen if encapsulation of BIG TCP will be
supported in the future.

The fix is to place the whole encapsulation branch after the BIG TCP code
block. This way, if encapsulation of BIG TCP will be supported,
inner_network_header will still be calculated with the correct value of
skb->data.
Also, by arranging the code that way, the optimisation does not
add an additional branch.

Signed-off-by: Richard Gobert <richardbgobert@gmail.com>
---
 include/net/gro.h      |  9 +++++++++
 net/ethernet/eth.c     | 14 +++++++++++---
 net/ipv6/ip6_offload.c | 20 +++++++++++++++-----
 3 files changed, 35 insertions(+), 8 deletions(-)

Comments

Eric Dumazet March 13, 2023, 4:51 p.m. UTC | #1 
On Mon, Mar 13, 2023 at 9:46 AM Richard Gobert <richardbgobert@gmail.com> wrote:
>
> Currently the IPv6 extension headers are parsed twice: first in
> ipv6_gro_receive, and then again in ipv6_gro_complete.
>
> By using the new ->transport_proto field, and also storing the size of the
> network header, we can avoid parsing extension headers a second time in
> ipv6_gro_complete (which saves multiple memory dereferences and conditional
> checks inside ipv6_exthdrs_len for a varying amount of extension headers in
> IPv6 packets).
>
> The implementation had to handle both inner and outer layers in case of
> encapsulation (as they can't use the same field). I've applied a similar
> optimisation to Ethernet.
>
> Performance tests for TCP stream over IPv6 with a varying amount of
> extension headers demonstrate throughput improvement of ~0.7%.
>
> In addition, I fixed a potential future problem:

I would remove all this block.

We fix current problems, not future hypothetical ones.

>  - The call to skb_set_inner_network_header at the beginning of
>    ipv6_gro_complete calculates inner_network_header based on skb->data by
>    calling skb_set_inner_network_header, and setting it to point to the
>    beginning of the ip header.
>  - If a packet is going to be handled by BIG TCP, the following code block
>    is going to shift the packet header, and skb->data is going to be
>    changed as well.
>
> When the two flows are combined, inner_network_header will point to the
> wrong place - which might happen if encapsulation of BIG TCP will be
> supported in the future.
>
> The fix is to place the whole encapsulation branch after the BIG TCP code
> block. This way, if encapsulation of BIG TCP will be supported,
> inner_network_header will still be calculated with the correct value of
> skb->data.

We do not support encapsulated BIG TCP yet.
We will do this later, and whoever does it will make sure to also support GRO.

> Also, by arranging the code that way, the optimisation does not
> add an additional branch.
>
> Signed-off-by: Richard Gobert <richardbgobert@gmail.com>
> ---
>

Can you give us a good explanation of why extension headers are used exactly ?

I am not sure we want to add code to GRO for something that 99.99% of
us do not use.
Richard Gobert March 14, 2023, 3:55 p.m. UTC | #2 
> >
> > Currently the IPv6 extension headers are parsed twice: first in
> > ipv6_gro_receive, and then again in ipv6_gro_complete.
> >
> > By using the new ->transport_proto field, and also storing the size of the
> > network header, we can avoid parsing extension headers a second time in
> > ipv6_gro_complete (which saves multiple memory dereferences and conditional
> > checks inside ipv6_exthdrs_len for a varying amount of extension headers in
> > IPv6 packets).
> >
> > The implementation had to handle both inner and outer layers in case of
> > encapsulation (as they can't use the same field). I've applied a similar
> > optimisation to Ethernet.
> >
> > Performance tests for TCP stream over IPv6 with a varying amount of
> > extension headers demonstrate throughput improvement of ~0.7%.
> >
> > In addition, I fixed a potential future problem:
> 
> I would remove all this block.
> 
> We fix current problems, not future hypothetical ones.
> 

I agree, I did it primarily to avoid an additional branch (the logic
remains exactly the same). I'll remove this part from the commit message.


> >  - The call to skb_set_inner_network_header at the beginning of
> >    ipv6_gro_complete calculates inner_network_header based on skb->data by
> >    calling skb_set_inner_network_header, and setting it to point to the
> >    beginning of the ip header.
> >  - If a packet is going to be handled by BIG TCP, the following code block
> >    is going to shift the packet header, and skb->data is going to be
> >    changed as well.
> >
> > When the two flows are combined, inner_network_header will point to the
> > wrong place - which might happen if encapsulation of BIG TCP will be
> > supported in the future.
> >
> > The fix is to place the whole encapsulation branch after the BIG TCP code
> > block. This way, if encapsulation of BIG TCP will be supported,
> > inner_network_header will still be calculated with the correct value of
> > skb->data.
> 
> We do not support encapsulated BIG TCP yet.
> We will do this later, and whoever does it will make sure to also support GRO.
> 
> > Also, by arranging the code that way, the optimisation does not
> > add an additional branch.
> >
> > Signed-off-by: Richard Gobert <richardbgobert@gmail.com>
> > ---
> >
> 
> Can you give us a good explanation of why extension headers are used exactly ?
> 
> I am not sure we want to add code to GRO for something that 99.99% of
> us do not use.

IMO, some common use cases that will benefit from this patch are:
- Parsing of BIG TCP packets which include a hbh ext hdr.
- dstopts and routing ext hdrs that are used for Mobile IPv6 features.

Generally, when a packet includes ext hdrs we will avoid the recalculation
of the ext hdrs len. When there are no ext hdrs, we will not call the
ipv6_exthdrs_len function so the performance isn't negatively impacted
(potentially even saving some opcodes in ipv6_exthdrs_len).
diff mbox series

Patch

diff --git a/include/net/gro.h b/include/net/gro.h
index 7b47dd6ce94f..35f60ea99f6c 100644
--- a/include/net/gro.h
+++ b/include/net/gro.h
@@ -86,6 +86,15 @@  struct napi_gro_cb {
 
 	/* used to support CHECKSUM_COMPLETE for tunneling protocols */
 	__wsum	csum;
+
+	/* Used in ipv6_gro_receive() */
+	u16	network_len;
+
+	/* Used in eth_gro_receive() */
+	__be16	network_proto;
+
+	/* Used in ipv6_gro_receive() */
+	u8	transport_proto;
 };
 
 #define NAPI_GRO_CB(skb) ((struct napi_gro_cb *)(skb)->cb)
diff --git a/net/ethernet/eth.c b/net/ethernet/eth.c
index 2edc8b796a4e..c2b77d9401e4 100644
--- a/net/ethernet/eth.c
+++ b/net/ethernet/eth.c
@@ -439,6 +439,9 @@  struct sk_buff *eth_gro_receive(struct list_head *head, struct sk_buff *skb)
 		goto out;
 	}
 
+	if (!NAPI_GRO_CB(skb)->encap_mark)
+		NAPI_GRO_CB(skb)->network_proto = type;
+
 	skb_gro_pull(skb, sizeof(*eh));
 	skb_gro_postpull_rcsum(skb, eh, sizeof(*eh));
 
@@ -455,13 +458,18 @@  EXPORT_SYMBOL(eth_gro_receive);
 
 int eth_gro_complete(struct sk_buff *skb, int nhoff)
 {
-	struct ethhdr *eh = (struct ethhdr *)(skb->data + nhoff);
-	__be16 type = eh->h_proto;
 	struct packet_offload *ptype;
+	struct ethhdr *eh;
 	int err = -ENOSYS;
+	__be16 type;
 
-	if (skb->encapsulation)
+	if (skb->encapsulation) {
+		eh = (struct ethhdr *)(skb->data + nhoff);
 		skb_set_inner_mac_header(skb, nhoff);
+		type = eh->h_proto;
+	} else {
+		type = NAPI_GRO_CB(skb)->network_proto;
+	}
 
 	ptype = gro_find_complete_by_type(type);
 	if (ptype != NULL)
diff --git a/net/ipv6/ip6_offload.c b/net/ipv6/ip6_offload.c
index 00dc2e3b0184..6e3a923ad573 100644
--- a/net/ipv6/ip6_offload.c
+++ b/net/ipv6/ip6_offload.c
@@ -232,6 +232,11 @@  INDIRECT_CALLABLE_SCOPE struct sk_buff *ipv6_gro_receive(struct list_head *head,
 	flush--;
 	nlen = skb_network_header_len(skb);
 
+	if (!NAPI_GRO_CB(skb)->encap_mark) {
+		NAPI_GRO_CB(skb)->transport_proto = proto;
+		NAPI_GRO_CB(skb)->network_len = nlen;
+	}
+
 	list_for_each_entry(p, head, list) {
 		const struct ipv6hdr *iph2;
 		__be32 first_word; /* <Version:4><Traffic_Class:8><Flow_Label:20> */
@@ -324,10 +329,6 @@  INDIRECT_CALLABLE_SCOPE int ipv6_gro_complete(struct sk_buff *skb, int nhoff)
 	int err = -ENOSYS;
 	u32 payload_len;
 
-	if (skb->encapsulation) {
-		skb_set_inner_protocol(skb, cpu_to_be16(ETH_P_IPV6));
-		skb_set_inner_network_header(skb, nhoff);
-	}
 
 	payload_len = skb->len - nhoff - sizeof(*iph);
 	if (unlikely(payload_len > IPV6_MAXPLEN)) {
@@ -341,6 +342,7 @@  INDIRECT_CALLABLE_SCOPE int ipv6_gro_complete(struct sk_buff *skb, int nhoff)
 		skb->len += hoplen;
 		skb->mac_header -= hoplen;
 		skb->network_header -= hoplen;
+		NAPI_GRO_CB(skb)->network_len += hoplen;
 		iph = (struct ipv6hdr *)(skb->data + nhoff);
 		hop_jumbo = (struct hop_jumbo_hdr *)(iph + 1);
 
@@ -358,7 +360,15 @@  INDIRECT_CALLABLE_SCOPE int ipv6_gro_complete(struct sk_buff *skb, int nhoff)
 		iph->payload_len = htons(payload_len);
 	}
 
-	nhoff += sizeof(*iph) + ipv6_exthdrs_len(iph, &ops);
+	if (skb->encapsulation) {
+		skb_set_inner_protocol(skb, cpu_to_be16(ETH_P_IPV6));
+		skb_set_inner_network_header(skb, nhoff);
+		nhoff += sizeof(*iph) + ipv6_exthdrs_len(iph, &ops);
+	} else {
+		ops = rcu_dereference(inet6_offloads[NAPI_GRO_CB(skb)->transport_proto]);
+		nhoff += NAPI_GRO_CB(skb)->network_len;
+	}
+
 	if (WARN_ON(!ops || !ops->callbacks.gro_complete))
 		goto out;