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

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Date:	Tue, 7 Jan 2014 22:37:54 +0200
From:	Or Gerlitz <or.gerlitz@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Or Gerlitz <ogerlitz@...lanox.com>, Jerry Chu <hkchu@...gle.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 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
which means the NIC has some support for doing RX checksum of
encapsulated packets. Per the current convension, in that case the NIC
RX code has to set skb->encapsulation see 6a674e9c75b17 "net: Add
support for hardware-offloaded encapsulation" this convension is
implemented in the current drivers that have HW offloads for
encapsulated packets (bnx2x, i40e and mlx4)


>
> You access udp_offloads[XXX], with XXX being in 0..65535 range, right ?
>
>
>> As for encapsulated packets, when you say random ports, are you
>> referring to a router which has multiple udp encapsulating protocols
>> where each uses different udp port? for this case and also to reduce
>> the memory footprint, we can use lookup in a list as done for the L2
>> protocols gro handlers in the list_for_each loop of dev_gro_receive(),
>> makes sense?
>
> I am speaking of a normal router, running linux kernel, and having
> GRO/TSO enabled.
>
> If each incoming UDP packet has to access one extra cache line in a
> 512KB array, its likely to be an extra cache line miss, if UDP dest
> port is mostly random (compared to ports used by very recent UDP
> packets)
>
>
>
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