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Date: Thu, 25 Jun 2015 19:57:32 -0700
From: Tom Herbert <tom@...bertland.com>
To: Ramu Ramamurthy <sramamur@...ux.vnet.ibm.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 Thu, Jun 25, 2015 at 6:06 PM, Ramu Ramamurthy
<sramamur@...ux.vnet.ibm.com> wrote:
> 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.
>
That may be, but this change would affect all uses of GRO with UDP
encapsulation not just for intel 10G NICs. For instance, pushing a lot
of checksum calculation into the napi for a single queue device could
overwhelm the corresponding CPU-- this is the motivation for the
restriction in the first place. We need to do a little more diligence
here.
Can you please provide more details about your tests and configuration
(# of flows, #queues, etc.). Also, please try enabling UDP checksum
this should eliminate need for checksum computation on the receiver
and allow GRO to be used. Enabling RCO should then eliminate checksum
computation on the host.
Thanks,
Tom
> 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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>
>
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