Subject: Re: [PATCH] - vxlan: gro not effective for intel 82599

From: Ramu Ramamurthy <[email-masked]>
Date: Thu, 25 Jun 2015 18:06:08 -0700

On 2015-06-25 17:20, Tom Herbert wrote:
> On Thu, Jun 25, 2015 at 5:03 PM, Ramu Ramamurthy
> <[email-masked]> 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 <[email-masked]>
>> ---
>>  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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