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Date: Thu, 25 Jun 2015 18:06:08 -0700
From: Ramu Ramamurthy <sramamur@...ux.vnet.ibm.com>
To: Tom Herbert <tom@...bertland.com>
Cc: "David S. Miller" <davem@...emloft.net>,
Tom Herbert <therbert@...gle.com>,
Jiri Benc <jbenc@...hat.com>, James Morris <jmorris@...ei.org>,
Linux Kernel Network Developers <netdev@...r.kernel.org>,
pradeeps@...ux.vnet.ibm.com, jkidambi@...ibm.com
Subject: Re: [PATCH] - vxlan: gro not effective for intel 82599
On 2015-06-25 17:20, Tom Herbert wrote:
> On Thu, Jun 25, 2015 at 5:03 PM, Ramu Ramamurthy
> <sramamur@...ux.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.ramamurthy@...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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