Re: [PATCH net-next] net: avoid unneeded UDP L4 and fraglist GSO resegmentation

[Alexander Lobakin]

From: Alexander Lobakin <alobakin@xxxxx>
Date: Sat, 31 Oct 2020 10:31:31 +0000

> On Saturday, 31 October 2020, 2:12, Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx> wrote:
>
> Hi Willem,
>
>> On Fri, Oct 30, 2020 at 2:33 PM Alexander Lobakin alobakin@xxxxx wrote:
>>
>>> Commit 9fd1ff5d2ac7 ("udp: Support UDP fraglist GRO/GSO.") added a support
>>> for fraglist UDP L4 and fraglist GSO not only for local traffic, but also
>>> for forwarding. This works well on simple setups, but when any logical
>>> netdev (e.g. VLAN) is present, kernel stack always performs software
>>> resegmentation which actually kills the performance.
>>> Despite the fact that no mainline drivers currently supports fraglist GSO,
>>> this should and can be easily fixed by adding UDP L4 and fraglist GSO to
>>> the list of GSO types that can be passed-through the logical interfaces
>>> (NETIF_F_GSO_SOFTWARE). After this change, no resegmentation occurs (if
>>> a particular driver supports and advertises this), and the performance
>>> goes on par with e.g. 1:1 forwarding.
>>> The only logical netdevs that seem to be unaffected to this are bridge
>>> interfaces, as their code uses full NETIF_F_GSO_MASK.
>>>
>>> Tested on MIPS32 R2 router board with a WIP NIC driver in VLAN NAT:
>>> 20 Mbps baseline, 1 Gbps / link speed with this patch.
>>>
>>> Signed-off-by: Alexander Lobakin alobakin@xxxxx
>>> ------------------------------------------------
>>> include/linux/netdev_features.h | 4 ++--
>>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>> diff --git a/include/linux/netdev_features.h b/include/linux/netdev_feautres.h
>>> index 0b17c4322b09..934de56644e7 100644
>>> --- a/include/linux/netdev_features.h
>>> +++ b/include/linux/netdev_features.h
>>> @@ -207,8 +207,8 @@ static inline int find_next_netdev_feature(u64 feature, unsigned long start)
>>> NETIF_F_FSO)
>>> /* List of features with software fallbacks. */
>>> -#define NETIF_F_GSO_SOFTWARE (NETIF_F_ALL_TSO | \
>>> -                                  NETIF_F_GSO_SCTP)
>>> +#define NETIF_F_GSO_SOFTWARE (NETIF_F_ALL_TSO | NETIF_F_GSO_SCTP | \
>>> -                                  NETIF_F_GSO_UDP_L4 | NETIF_F_GSO_FRAGLIST)
>>
>> What exactly do you mean by resegmenting?
>
> I mean pts 5-6 from the full path:
> 1. Our NIC driver advertises a support for fraglists, GSO UDP L4, GSO fraglists.
> 2. User enables fraglisted GRO via Ethtool.
> 3. GRO subsystem receives UDP frames from driver and merges the packets into
>    fraglisted GSO skb(s).
> 4. Networking stack queues it up for xmitting.
> 5. Virtual device like VLAN doesn't advertise a support for GSO UDP L4 and
>    GSO fraglists, so skb_gso_check() doesn't allow to pass this skb as is to
>    the real driver.
> 6. Kernel then has to form a bunch of regular UDP skbs from that one and pass
>    it to the driver instead. This fallback is *extremely* slow for any GSO types,
>    but especially for GSO fraglists.
> 7. All further processing performs with a series of plain UDP skbs, and the
>    driver gets it one-by-one, despite that it supports UDP L4 and fraglisted GSO.
>
> That's not OK because:
> a) logical/virtual netdevs like VLANs, bridges etc. should pass GSO skbs as is;
> b) even if the final driver doesn't support such type of GSO, this software
>    resegmenting should be performed right before it, not in the middle of
>    processing -- I think I even saw that note somewhere in kernel documentation,
>    and it's totally reasonable in terms of performance.
>
>> I think it is fine to reenable this again, now that UDP sockets will
>> segment unexpected UDP GSO packets that may have looped. We previously
>> added general software support in commit 83aa025f535f ("udp: add gso
>> support to virtual devices"). Then reduced its scope to egress only in
>> 8eea1ca82be9 ("gso: limit udp gso to egress-only virtual devices") to
>> handle that edge case.

Regarding bonding and teaming: I think they should also use
NETIF_F_GSO_SOFTWARE mask, not NETIF_F_ALL_TSO, as SCTP also has
a software fallback. This way we could also remove a separate
advertising of NETIF_F_GSO_UDP_L4, as it will be included in the first.

So, if this one:
1. Add NETIF_F_GSO_UDP_L4 and NETIF_F_GSO_FRAGLIST to
   NETIF_F_GSO_SOFTWARE;
2. Change bonding and teaming features mask from NETIF_F_ALL_TSO |
   NETIF_F_GSO_UDP_L4 to NETIF_F_GSO_SOFTWARE;
3. Check that every virtual netdev has NETIF_F_GSO_SOFTWARE _or_
   NETIF_F_GSO_MASK in its advertising.

is fine for everyone, I'll publish more appropriate and polished v2 soon.

>> If we can enable for all virtual devices again, we could revert those
>> device specific options.
>
> Thanks,
> Al

Al