Re: [RFC PATCH v2 bpf-next 00/15] xdp_flow: Flow offload to XDP

[Toshiaki Makita <toshiaki.makita1@gmail.com>]

On 2019/10/19 0:22, John Fastabend wrote:
> Toshiaki Makita wrote:
>> This is a PoC for an idea to offload flow, i.e. TC flower and nftables,
>> to XDP.
>>
> 
> I've only read the cover letter so far but...

Thank you for reading this long cover letter.

> 
>> * Motivation
>>
>> The purpose is to speed up flow based network features like TC flower and
>> nftables by making use of XDP.
>>
>> I chose flow feature because my current interest is in OVS. OVS uses TC
>> flower to offload flow tables to hardware, so if TC can offload flows to
>> XDP, OVS also can be offloaded to XDP.
> 
> This adds a non-trivial amount of code and complexity so I'm
> critical of the usefulness of being able to offload TC flower to
> XDP when userspace can simply load an XDP program.
> 
> Why does OVS use tc flower at all if XDP is about 5x faster using
> your measurements below? Rather than spend energy adding code to
> a use case that as far as I can tell is narrowly focused on offload
> support can we enumerate what is missing on XDP side that blocks
> OVS from using it directly?

I think nothing is missing for direct XDP use, as long as XDP datapath
only partially supports OVS flow parser/actions like xdp_flow.
The point is to avoid duplicate effort when someone wants to use XDP
through TC flower or nftables transparently.

> Additionally for hardware that can
> do XDP/BPF offload you will get the hardware offload for free.

This is not necessary as OVS already uses TC flower to offload flows.

> Yes I know XDP is bytecode and you can't "offload" bytecode into
> a flow based interface likely backed by a tcam but IMO that doesn't
> mean we should leak complexity into the kernel network stack to
> fix this. Use the tc-flower for offload only (it has support for
> this) if you must and use the best (in terms of Mpps) software
> interface for your software bits. And if you want auto-magic
> offload support build hardware with BPF offload support.
> 
> In addition by using XDP natively any extra latency overhead from
> bouncing calls through multiple layers would be removed.

To some extent yes, but not completely. Flow insertion from userspace
triggered by datapath upcall is necessary regardless of whether we use
TC or not.

>> When TC flower filter is offloaded to XDP, the received packets are
>> handled by XDP first, and if their protocol or something is not
>> supported by the eBPF program, the program returns XDP_PASS and packets
>> are passed to upper layer TC.
>>
>> The packet processing flow will be like this when this mechanism,
>> xdp_flow, is used with OVS.
> 
> Same as obove just cross out the 'TC flower' box and add support
> for your missing features to 'XDP prog' box. Now you have less
> code to maintain and less bugs and aren't pushing packets through
> multiple hops in a call chain.

If we cross out TC then we would need similar code in OVS userspace.
In total I don't think it would be less code to maintain.

> 
>>
>>   +-------------+
>>   | openvswitch |
>>   |    kmod     |
>>   +-------------+
>>          ^
>>          | if not match in filters (flow key or action not supported by TC)
>>   +-------------+
>>   |  TC flower  |
>>   +-------------+
>>          ^
>>          | if not match in flow tables (flow key or action not supported by XDP)
>>   +-------------+
>>   |  XDP prog   |
>>   +-------------+
>>          ^
>>          | incoming packets
>>
>> Of course we can directly use TC flower without OVS to speed up TC.
> 
> huh? TC flower is part of TC so not sure what 'speed up TC' means. I
> guess this means using tc flower offload to xdp prog would speed up
> general tc flower usage as well?

Yes.

> 
> But again if we are concerned about Mpps metrics just write the XDP
> program directly.

I guess you mean any Linux users who want TC-like flow handling should develop
their own XDP programs? (sorry if I misunderstand you.)
I want to avoid such a situation. The flexibility of eBPF/XDP is nice and it's
good to have any program each user wants, but not every sysadmin can write low
level good performance programs like us. For typical use-cases like flow handling
easy use of XDP through existing kernel interface (here TC) is useful IMO.

> 
...
>> * About alternative userland (ovs-vswitchd etc.) implementation
>>
>> Maybe a similar logic can be implemented in ovs-vswitchd offload
>> mechanism, instead of adding code to kernel. I just thought offloading
>> TC is more generic and allows wider usage with direct TC command.
>>
>> For example, considering that OVS inserts a flow to kernel only when
>> flow miss happens in kernel, we can in advance add offloaded flows via
>> tc filter to avoid flow insertion latency for certain sensitive flows.
>> TC flower usage without using OVS is also possible.
> 
> I argue to cut tc filter out entirely and then I think non of this
> is needed.

Not correct. Even with native XDP use, multiple map lookup/modification
from userspace is necessary for flow miss handling, which will lead to
some latency.

And there are other use-cases for direct TC use, like packet drop or
redirection for certain flows.

> 
>>
>> Also as written above nftables can be offloaded to XDP with this
>> mechanism as well.
> 
> Or same argument use XDP directly.

I'm thinking it's useful for sysadmins to be able to use XDP through
existing kernel interfaces.

> 
>>
>> Another way to achieve this from userland is to add notifications in
>> flow_offload kernel code to inform userspace of flow addition and
>> deletion events, and listen them by a deamon which in turn loads eBPF
>> programs, attach them to XDP, and modify eBPF maps. Although this may
>> open up more use cases, I'm not thinking this is the best solution
>> because it requires emulation of kernel behavior as an offload engine
>> but flow related code is heavily changing which is difficult to follow
>> from out of tree.
> 
> So if everything was already in XDP why would we need these
> notifications? I think a way to poll on a map from user space would
> be a great idea e.g. everytime my XDP program adds a flow to my
> hash map wake up my userspace agent with some ctx on what was added or
> deleted so I can do some control plane logic.

I was talking about TC emulation above, so map notification is not related
to this problem, although it may be a nice feature.

> 
> [...]
> 
> Lots of code churn...

Note that most of it is TC offload driver implementation. So it should add
little complexity to network/XDP/TC core.

> 
>>   24 files changed, 2864 insertions(+), 30 deletions(-)
> 
> Thanks,
> John
> 

Toshiaki Makita