Re: [PATCH net-next] ibmveth: v1 calculate correct gso_size and set gso_type

[Jonathan Maxwell]

On Thu, Nov 3, 2016 at 8:40 AM, Brian King  wrote:
> On 10/27/2016 10:26 AM, Eric Dumazet wrote:
>> On Wed, 2016-10-26 at 11:09 +1100, Jon Maxwell wrote:
>>> We recently encountered a bug where a few customers using ibmveth on the
>>> same LPAR hit an issue where a TCP session hung when large receive was
>>> enabled. Closer analysis revealed that the session was stuck because the
>>> one side was advertising a zero window repeatedly.
>>>
>>> We narrowed this down to the fact the ibmveth driver did not set gso_size
>>> which is translated by TCP into the MSS later up the stack. The MSS is
>>> used to calculate the TCP window size and as that was abnormally large,
>>> it was calculating a zero window, even although the sockets receive buffer
>>> was completely empty.
>>>
>>> We were able to reproduce this and worked with IBM to fix this. Thanks Tom
>>> and Marcelo for all your help and review on this.
>>>
>>> The patch fixes both our internal reproduction tests and our customers 
>>> tests.
>>>
>>> Signed-off-by: Jon Maxwell 
>>> ---
>>>  drivers/net/ethernet/ibm/ibmveth.c | 20 
>>>  1 file changed, 20 insertions(+)
>>>
>>> diff --git a/drivers/net/ethernet/ibm/ibmveth.c 
>>> b/drivers/net/ethernet/ibm/ibmveth.c
>>> index 29c05d0..c51717e 100644
>>> --- a/drivers/net/ethernet/ibm/ibmveth.c
>>> +++ b/drivers/net/ethernet/ibm/ibmveth.c
>>> @@ -1182,6 +1182,8 @@ static int ibmveth_poll(struct napi_struct *napi, int 
>>> budget)
>>>  int frames_processed = 0;
>>>  unsigned long lpar_rc;
>>>  struct iphdr *iph;
>>> +bool large_packet = 0;
>>> +u16 hdr_len = ETH_HLEN + sizeof(struct tcphdr);
>>>
>>>  restart_poll:
>>>  while (frames_processed < budget) {
>>> @@ -1236,10 +1238,28 @@ static int ibmveth_poll(struct napi_struct *napi, 
>>> int budget)
>>>  iph->check = 0;
>>>  iph->check = 
>>> ip_fast_csum((unsigned char *)iph, iph->ihl);
>>>  adapter->rx_large_packets++;
>>> +large_packet = 1;
>>>  }
>>>  }
>>>  }
>>>
>>> +if (skb->len > netdev->mtu) {
>>> +iph = (struct iphdr *)skb->data;
>>> +if (be16_to_cpu(skb->protocol) == ETH_P_IP &&
>>> +iph->protocol == IPPROTO_TCP) {
>>> +hdr_len += sizeof(struct iphdr);
>>> +skb_shinfo(skb)->gso_type = 
>>> SKB_GSO_TCPV4;
>>> +skb_shinfo(skb)->gso_size = 
>>> netdev->mtu - hdr_len;
>>> +} else if (be16_to_cpu(skb->protocol) == 
>>> ETH_P_IPV6 &&
>>> +   iph->protocol == IPPROTO_TCP) {
>>> +hdr_len += sizeof(struct ipv6hdr);
>>> +skb_shinfo(skb)->gso_type = 
>>> SKB_GSO_TCPV6;
>>> +skb_shinfo(skb)->gso_size = 
>>> netdev->mtu - hdr_len;
>>> +}
>>> +if (!large_packet)
>>> +adapter->rx_large_packets++;
>>> +}
>>> +
>>>
>>
>> This might break forwarding and PMTU discovery.
>>
>> You force gso_size to device mtu, regardless of real MSS used by the TCP
>> sender.
>>
>> Don't you have the MSS provided in RX descriptor, instead of guessing
>> the value ?
>
> We've had some further discussions on this with the Virtual I/O Server (VIOS)
> development team. The large receive aggregation in the VIOS (AIX based) is 
> actually
> being done by software in the VIOS. What they may be able to do is when 
> performing
> this aggregation, they could look at the packet lengths of all the packets 
> being
> aggregated and take the largest packet size within the aggregation unit, 
> minus the
> header length and return that to the virtual ethernet client which we could 
> then stuff
> into gso_size. They are currently assessing how feasible this would be to do 
> and whether
> it would impact other bits of the code. However, assuming this does end up 
> being an option,
> would this address the concerns here or is that going to break something else 
> I'm
> not thinking of?

I was discussing this with a colleague and although this is better than
what we have so far. We wonder if there could be a corner case where
it ends up with a smaller value than the current MSS. For example if
the application sent a burst of small TCP packets with the PUSH
bit set. In that case they may not be coalesced by GRO. The VIOS could
probably be coded to detect that condition and use the previous MSS.
But that may not necessarily be the current MSS.

The ibmveth driver passes 

Re: [PATCH net-next] ibmveth: v1 calculate correct gso_size and set gso_type

[Jonathan Maxwell]

On Fri, Oct 28, 2016 at 5:08 AM, Eric Dumazet  wrote:
> On Thu, 2016-10-27 at 12:54 -0500, Thomas Falcon wrote:
>> On 10/27/2016 10:26 AM, Eric Dumazet wrote:
>> > On Wed, 2016-10-26 at 11:09 +1100, Jon Maxwell wrote:
>> >> We recently encountered a bug where a few customers using ibmveth on the
>> >> same LPAR hit an issue where a TCP session hung when large receive was
>> >> enabled. Closer analysis revealed that the session was stuck because the
>> >> one side was advertising a zero window repeatedly.
>> >>
>> >> We narrowed this down to the fact the ibmveth driver did not set gso_size
>> >> which is translated by TCP into the MSS later up the stack. The MSS is
>> >> used to calculate the TCP window size and as that was abnormally large,
>> >> it was calculating a zero window, even although the sockets receive buffer
>> >> was completely empty.
>> >>
>> >> We were able to reproduce this and worked with IBM to fix this. Thanks Tom
>> >> and Marcelo for all your help and review on this.
>> >>
>> >> The patch fixes both our internal reproduction tests and our customers 
>> >> tests.
>> >>
>> >> Signed-off-by: Jon Maxwell 
>> >> ---
>> >>  drivers/net/ethernet/ibm/ibmveth.c | 20 
>> >>  1 file changed, 20 insertions(+)
>> >>
>> >> diff --git a/drivers/net/ethernet/ibm/ibmveth.c 
>> >> b/drivers/net/ethernet/ibm/ibmveth.c
>> >> index 29c05d0..c51717e 100644
>> >> --- a/drivers/net/ethernet/ibm/ibmveth.c
>> >> +++ b/drivers/net/ethernet/ibm/ibmveth.c
>> >> @@ -1182,6 +1182,8 @@ static int ibmveth_poll(struct napi_struct *napi, 
>> >> int budget)
>> >>int frames_processed = 0;
>> >>unsigned long lpar_rc;
>> >>struct iphdr *iph;
>> >> +  bool large_packet = 0;
>> >> +  u16 hdr_len = ETH_HLEN + sizeof(struct tcphdr);
>> >>
>> >>  restart_poll:
>> >>while (frames_processed < budget) {
>> >> @@ -1236,10 +1238,28 @@ static int ibmveth_poll(struct napi_struct *napi, 
>> >> int budget)
>> >>iph->check = 0;
>> >>iph->check = 
>> >> ip_fast_csum((unsigned char *)iph, iph->ihl);
>> >>adapter->rx_large_packets++;
>> >> +  large_packet = 1;
>> >>}
>> >>}
>> >>}
>> >>
>> >> +  if (skb->len > netdev->mtu) {
>> >> +  iph = (struct iphdr *)skb->data;
>> >> +  if (be16_to_cpu(skb->protocol) == ETH_P_IP &&
>> >> +  iph->protocol == IPPROTO_TCP) {
>> >> +  hdr_len += sizeof(struct iphdr);
>> >> +  skb_shinfo(skb)->gso_type = 
>> >> SKB_GSO_TCPV4;
>> >> +  skb_shinfo(skb)->gso_size = 
>> >> netdev->mtu - hdr_len;
>> >> +  } else if (be16_to_cpu(skb->protocol) == 
>> >> ETH_P_IPV6 &&
>> >> + iph->protocol == IPPROTO_TCP) {
>> >> +  hdr_len += sizeof(struct ipv6hdr);
>> >> +  skb_shinfo(skb)->gso_type = 
>> >> SKB_GSO_TCPV6;
>> >> +  skb_shinfo(skb)->gso_size = 
>> >> netdev->mtu - hdr_len;
>> >> +  }
>> >> +  if (!large_packet)
>> >> +  adapter->rx_large_packets++;
>> >> +  }
>> >> +
>> >>
>> > This might break forwarding and PMTU discovery.
>> >
>> > You force gso_size to device mtu, regardless of real MSS used by the TCP
>> > sender.
>> >
>> > Don't you have the MSS provided in RX descriptor, instead of guessing
>> > the value ?
>> >
>> >
>> >
>> The MSS is not always available unfortunately, so this is the best solution 
>> there is at the moment.
>
> Hmm... then what about skb_shinfo(skb)->gso_segs ?
>
> ip_rcv() for example has :
>
> __IP_ADD_STATS(net,
>IPSTATS_MIB_NOECTPKTS + (iph->tos & INET_ECN_MASK),
>max_t(unsigned short, 1, skb_shinfo(skb)->gso_segs));
>
>

Okay thanks Eric. As the MSS is the main concern, let me work with the
team here and see if we find can a better way to do this. Will take care of
the other points you raised at the same time.

>
> Also prefer : (skb->protocol == htons(ETH_P_IP)) tests
>
> And the ipv6 test is wrong :
>
> } else if (be16_to_cpu(skb->protocol) == ETH_P_IPV6 &&
>iph->protocol == IPPROTO_TCP) {
>
>
> Since iph is a pointer to ipv4 iphdr .
>
>
>

Re: [PATCH net-next] ibmveth: calculate correct gso_size and set gso_type

[Jonathan Maxwell]

>> + u16 hdr_len = ETH_HLEN + sizeof(struct tcphdr);

> Compiler may optmize this, but maybe move hdr_len to [*] ?>

There are other places in the stack where a u16 is used for the
same purpose. So I'll rather stick to that convention.

I'll make the other formatting changes you suggested and
resubmit as v1.

Thanks

Jon

On Tue, Oct 25, 2016 at 9:31 PM, Marcelo Ricardo Leitner
 wrote:
> On Tue, Oct 25, 2016 at 04:13:41PM +1100, Jon Maxwell wrote:
>> We recently encountered a bug where a few customers using ibmveth on the
>> same LPAR hit an issue where a TCP session hung when large receive was
>> enabled. Closer analysis revealed that the session was stuck because the
>> one side was advertising a zero window repeatedly.
>>
>> We narrowed this down to the fact the ibmveth driver did not set gso_size
>> which is translated by TCP into the MSS later up the stack. The MSS is
>> used to calculate the TCP window size and as that was abnormally large,
>> it was calculating a zero window, even although the sockets receive buffer
>> was completely empty.
>>
>> We were able to reproduce this and worked with IBM to fix this. Thanks Tom
>> and Marcelo for all your help and review on this.
>>
>> The patch fixes both our internal reproduction tests and our customers tests.
>>
>> Signed-off-by: Jon Maxwell 
>> ---
>>  drivers/net/ethernet/ibm/ibmveth.c | 19 +++
>>  1 file changed, 19 insertions(+)
>>
>> diff --git a/drivers/net/ethernet/ibm/ibmveth.c 
>> b/drivers/net/ethernet/ibm/ibmveth.c
>> index 29c05d0..3028c33 100644
>> --- a/drivers/net/ethernet/ibm/ibmveth.c
>> +++ b/drivers/net/ethernet/ibm/ibmveth.c
>> @@ -1182,6 +1182,8 @@ static int ibmveth_poll(struct napi_struct *napi, int 
>> budget)
>>   int frames_processed = 0;
>>   unsigned long lpar_rc;
>>   struct iphdr *iph;
>> + bool large_packet = 0;
>> + u16 hdr_len = ETH_HLEN + sizeof(struct tcphdr);
>
> Compiler may optmize this, but maybe move hdr_len to [*] ?
>
>>
>>  restart_poll:
>>   while (frames_processed < budget) {
>> @@ -1236,10 +1238,27 @@ static int ibmveth_poll(struct napi_struct *napi, 
>> int budget)
>>   iph->check = 0;
>>   iph->check = 
>> ip_fast_csum((unsigned char *)iph, iph->ihl);
>>   adapter->rx_large_packets++;
>> + large_packet = 1;
>>   }
>>   }
>>   }
>>
>> + if (skb->len > netdev->mtu) {
>
> [*]
>
>> + iph = (struct iphdr *)skb->data;
>> + if (be16_to_cpu(skb->protocol) == ETH_P_IP && 
>> iph->protocol == IPPROTO_TCP) {
>
> The if line above is too long, should be broken in two.
>
>> + hdr_len += sizeof(struct iphdr);
>> + skb_shinfo(skb)->gso_type = 
>> SKB_GSO_TCPV4;
>> + skb_shinfo(skb)->gso_size = 
>> netdev->mtu - hdr_len;
>> + } else if (be16_to_cpu(skb->protocol) == 
>> ETH_P_IPV6 &&
>> + iph->protocol == IPPROTO_TCP) {
> ^
> And this one should start 3 spaces later, right below be16_
>
>   Marcelo
>
>> + hdr_len += sizeof(struct ipv6hdr);
>> + skb_shinfo(skb)->gso_type = 
>> SKB_GSO_TCPV6;
>> + skb_shinfo(skb)->gso_size = 
>> netdev->mtu - hdr_len;
>> + }
>> + if (!large_packet)
>> + adapter->rx_large_packets++;
>> + }
>> +
>>   napi_gro_receive(napi, skb);/* send it up */
>>
>>   netdev->stats.rx_packets++;
>> --
>> 1.8.3.1
>>