netdev - Re: [PATCH net-next V2 1/3] net: Add GRO support for UDP encapsulating protocols

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Date:	Wed, 8 Jan 2014 19:12:00 -0800
From:	Jerry Chu <hkchu@...gle.com>
To:	Or Gerlitz <ogerlitz@...lanox.com>
Cc:	Or Gerlitz <or.gerlitz@...il.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Yan Burman <yanb@...lanox.com>,
	Shlomo Pongratz <shlomop@...lanox.com>
Subject: Re: [PATCH net-next V2 1/3] net: Add GRO support for UDP
 encapsulating protocols

On Wed, Jan 8, 2014 at 12:02 AM, Or Gerlitz <ogerlitz@...lanox.com> wrote:
> On 08/01/2014 00:11, Jerry Chu wrote:
>>
>> On Tue, Jan 7, 2014 at 12:37 PM, Or Gerlitz <or.gerlitz@...il.com> wrote:
>>>
>>> On Tue, Jan 7, 2014 at 10:32 PM, Eric Dumazet <eric.dumazet@...il.com>
>>> wrote:
>>>>
>>>> On Tue, 2014-01-07 at 22:19 +0200, Or Gerlitz wrote:
>>>>>
>>>>> On Tue, Jan 7, 2014 at 6:33 PM, Eric Dumazet <eric.dumazet@...il.com>
>>>>> wrote:
>>>>>>
>>>>>> On Tue, 2014-01-07 at 17:29 +0200, Or Gerlitz wrote:
>>>>>>>
>>>>>>> +
>>>>>>> +#define MAX_UDP_PORT (1 << 16)
>>>>>>> +extern const struct net_offload __rcu *udp_offloads[MAX_UDP_PORT];
>>>>>>
>>>>>> Thats 512 KB of memory.
>>>>>> This will greatly impact forwarding performance of UDP packets with
>>>>>> random ports, and will increase kernel memory size for embedded
>>>>>> devices.
>>>>>
>>>>> Re forwarding, are you referring to the case where the forwarded
>>>>> packets are encapsulated? packets which are not encapusalted will be
>>>>> flushed in the gro receive handler (this went out by mistake in V2 but
>>>>> exists in V1)  if skb->encapsulation isn't set.
>>>>>
>>>> How do you know encapsulation must be tried for a given incoming
>>>> packet ? NIC do not magically sets skb->encapsulation I think...
>>>
>>> So here's the thing, per my understanding we want to GRO only received
>>> **encapsulated** packets whose checksum status is != CHECKSUM_NONE
>>
>> What's wrong with GRO'ing pkts whose csum == CHECKSUM_NONE?
>
>
> I am not sure, intuitively it sounds a bit wrong to me, empirically, it
> doesn't work for udp encapsulated  / vxlan
> traffic, I got drops from the tcp stack in tcp_rcv_established() -- if
> GRO-ed packets carry CHECKSUM_NONE
> we arrive to the csum_error label, which means that
> tcp_checksum_complete_user() failed for them

This is odd because if pkts have been aggregated successfully,
tcp4_gro_receive() should've skb_checksum() and turned CHECKSUM_NONE
into CHECKSUM_UNNECESSARY. (I think i've already tested this
case with my GRE-GRO patch on a NIC that sends up pkts w/ CHECKSUM_NONE.

But granted there are a lot of csum related bugs in the current code. I just
spent half a day scratching my head on a very low thruput number with
my GRE patch over a GRE tunnel w/ csum flag on. I just tracked it down
to be buggy TSO/GRE code that will produce bad csum on the tx side.

>
>
>
>> Also "udp_offload" is a little misleading - you are not trying to GRO UDP
>> pkts where UDP is the real transport. You are only trying to GRO UDP
>> encapped TCP pkts.
>
>
> Indeed -- however, I just plugged into what was there for GSO, e.g stack
> will not do GSO for plain UDP
> packets, only for those who encapsulate something the code that does this is
> udp_offloads.c -- any suggestion
> how to phrase/frame the change you envision?

There is already udpv4_offload for real udp gso (ufo) offload. How about
"udp_encap_offload" for your stuff?

>
>
>>
>
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