On 2015-06-25 17:20, Tom Herbert wrote:
On Thu, Jun 25, 2015 at 5:03 PM, Ramu Ramamurthy
<srama...@linux.vnet.ibm.com> wrote:
Problem:
-------
GRO is enabled on the interfaces in the following test,
but GRO does not take effect for vxlan-encapsulated tcp streams. The
root
cause of why GRO does not take effect is described below.
VM nic (mtu 1450)---bridge---vxlan----10Gb nic (intel 82599ES)-----|
VM nic (mtu 1450)---bridge---vxlan----10Gb nic (intel 82599ES)-----|
Because gro is not effective, the throughput for vxlan-encapsulated
tcp-stream is around 3 Gbps.
With the proposed patch, gro takes effect for vxlan-encapsulated tcp
streams,
and performance in the same test is around 8.6 Gbps.
Root Cause:
----------
At entry to udp4_gro_receive(), the gro parameters are set as follows:
skb->ip_summed == 0 (CHECKSUM_NONE)
NAPI_GRO_CB(skb)->csum_cnt == 0
NAPI_GRO_CB(skb)->csum_valid == 0
UDH header checksum is 0.
static struct sk_buff **udp4_gro_receive(struct sk_buff **head,
struct sk_buff *skb)
{
<snip>
if (skb_gro_checksum_validate_zero_check(skb, IPPROTO_UDP,
uh->check,
inet_gro_compute_pseudo))
This calls __skb_incr_checksum_unnecessary which sets
skb->ip_summed to CHECKSUM_UNNECESSARY
goto flush;
else if (uh->check)
skb_gro_checksum_try_convert(skb, IPPROTO_UDP,
uh->check,
inet_gro_compute_pseudo);
skip:
NAPI_GRO_CB(skb)->is_ipv6 = 0;
return udp_gro_receive(head, skb, uh);
}
struct sk_buff **udp_gro_receive(struct sk_buff **head, struct sk_buff
*skb,
struct udphdr *uh)
{
struct udp_offload_priv *uo_priv;
struct sk_buff *p, **pp = NULL;
struct udphdr *uh2;
unsigned int off = skb_gro_offset(skb);
int flush = 1;
if (NAPI_GRO_CB(skb)->udp_mark ||
(skb->ip_summed != CHECKSUM_PARTIAL &&
NAPI_GRO_CB(skb)->csum_cnt == 0 &&
!NAPI_GRO_CB(skb)->csum_valid))
goto out;
vxlan GRO gets skipped due to the above condition because
here,:
skb->ip_summed == CHECKSUM_UNNECESSARY
NAPI_GRO_CB(skb)->csum_cnt == 0
NAPI_GRO_CB(skb)->csum_valid == 0
There is no reason for skipping vxlan gro in the above combination of
conditions,
because, tcp4_gro_receive() validates the inner tcp checksum anyway !
Patch:
------
Signed-off-by: Ramu Ramamurthy <ramu.ramamur...@us.ibm.com>
---
net/ipv4/udp_offload.c | 1 +
1 files changed, 1 insertions(+), 0 deletions(-)
diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index f938616..17fc12b 100644
--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c
@@ -301,6 +301,7 @@ struct sk_buff **udp_gro_receive(struct sk_buff
**head,
struct sk_buff *skb,
if (NAPI_GRO_CB(skb)->udp_mark ||
(skb->ip_summed != CHECKSUM_PARTIAL &&
+ skb->ip_summed != CHECKSUM_UNNECESSARY &&
NAPI_GRO_CB(skb)->csum_cnt == 0 &&
!NAPI_GRO_CB(skb)->csum_valid))
goto out;
--
This isn't right. The CHECKSUM_UNNECESSARY only refers to the outer
checksum which is zero in this case so it is trivially unnecessary.
The inner checksum still needs to be computed on the host. By
convention, we do not do GRO if it is required to compute the inner
checksum (csum_cnt == 0 checks that). If we want to allow checksum
calculation to occur in the GRO path, meaning we understand the
ramifications and can show this is better for performance, then all
the checks about checksum here should be removed.
Isnt the inner checksum computed on the gro-path from tcp4_gro_receive()
as follows ?
This trace is from my testbed.
In my tests, I consistently get 8.5-9 Gbps with vxlan gro (inspite of
the added sw inner checksumming), whereas without vxlan GRO the
performance
drops down to 3Gbps or so. So, a significant performance benefit can be
gained
on intel 10G nics which are widely deployed. Hence the interest in
pursuing this or a modified patch.
vxlan_gro_receive <-udp4_gro_receive
ksoftirqd/1-94 [001] ..s. 11421.420280: __pskb_pull_tail
<-vxlan_gro_receive
ksoftirqd/1-94 [001] ..s. 11421.420280: skb_copy_bits
<-__pskb_pull_tail
ksoftirqd/1-94 [001] ..s. 11421.420280: __pskb_pull_tail
<-vxlan_gro_receive
ksoftirqd/1-94 [001] ..s. 11421.420281: skb_copy_bits
<-__pskb_pull_tail
ksoftirqd/1-94 [001] ..s. 11421.420281: gro_find_receive_by_type
<-vxlan_gro_receive
ksoftirqd/1-94 [001] ..s. 11421.420281: inet_gro_receive
<-vxlan_gro_receive
ksoftirqd/1-94 [001] ..s. 11421.420281: __pskb_pull_tail
<-inet_gro_receive
ksoftirqd/1-94 [001] ..s. 11421.420281: skb_copy_bits
<-__pskb_pull_tail
ksoftirqd/1-94 [001] ..s. 11421.420281: tcp4_gro_receive
<-inet_gro_receive
ksoftirqd/1-94 [001] ..s. 11421.420281:
__skb_gro_checksum_complete <-tcp4_gro_receive
ksoftirqd/1-94 [001] ..s. 11421.420281: skb_checksum
<-__skb_gro_checksum_complete
ksoftirqd/1-94 [001] ..s. 11421.420281: __skb_checksum
<-skb_checksum
ksoftirqd/1-94 [001] ..s1 11421.420281: csum_partial
<-csum_partial_ext
ksoftirqd/1-94 [001] ..s1 11421.420281: do_csum <-csum_partial
1.7.1
Notes:
-------
The above gro fix applies to all udp-encapsulation protocols (vxlan,
geneve)
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