Re: Flows! Offload them.

Neil Horman
2015-02-26T20:17:19Z

On Thu, Feb 26, 2015 at 07:23:36AM -0800, John Fastabend wrote:
> On 02/26/2015 05:33 AM, Thomas Graf wrote:
> > On 02/26/15 at 10:16am, Jiri Pirko wrote:
> >> Well, on netdev01, I believe that a consensus was reached that for every
> >> switch offloaded functionality there has to be an implementation in
> >> kernel.
> >
> > Agreed. This should not prevent the policy being driven from user
> > space though.
> >
> >> What John's Flow API originally did was to provide a way to
> >> configure hardware independently of kernel. So the right way is to
> >> configure kernel and, if hw allows it, to offload the configuration to hw.
> >>
> >> In this case, seems to me logical to offload from one place, that being
> >> TC. The reason is, as I stated above, the possible conversion from OVS
> >> datapath to TC.
> >
> > Offloading of TC definitely makes a lot of sense. I think that even in
> > that case you will already encounter independent configuration of
> > hardware and kernel. Example: The hardware provides a fixed, generic
> > function to push up to n bytes onto a packet. This hardware function
> > could be used to implement TC actions "push_vlan", "push_vxlan",
> > "push_mpls". You would you would likely agree that TC should make use
> > of such a function even if the hardware version is different from the
> > software version. So I don't think we'll have a 1:1 mapping for all
> > configurations, regardless of whether the how is decided in kernel or
> > user space.
>
> Just to expand slightly on this. I don't think you can get to a 1:1
> mapping here. One reason is hardware typically has a TCAM and limited
> size. So you need a _policy_ to determine when to push rules into the
> hardware. The kernel doesn't know when to do this and I don't believe
> its the kernel's place to start enforcing policy like this. One thing I likely
> need to do is get some more "worlds" in rocker so we aren't stuck only
> thinking about the infinite size OF_DPA world. The OF_DPA world is only
> one world and not a terribly flexible one at that when compared with the
> NPU folk. So minimally you need a flag to indicate rules go into hardware
> vs software.
>
> That said I think the bigger mismatch between software and hardware is
> you program it differently because the data structures are different. Maybe
> a u32 example would help. For parsing with u32 you might build a parse
> graph with a root and some leaf nodes. In hardware you want to collapse
> this down onto the hardware. I argue this is not a kernel task because
> there are lots of ways to do this and there are trade-offs made with
> respect to space and performance and which table to use when it could be
> handled by a set of tables. Another example is a virtual switch possibly
> OVS but we have others. The software does some "unmasking" (there term)
> before sending the rules into the software dataplane cache. Basically this
> means we can ignore priority in the hash lookup. However this is not how you
> would optimally use hardware. Maybe I should do another write up with
> some more concrete examples.
>
> There are also lots of use cases to _not_ have hardware and software in
> sync. A flag allows this.
>
> My only point is I think we need to allow users to optimally use there
> hardware either via 'tc' or my previous 'flow' tool. Actually in my
> opinion I still think its best to have both interfaces.
>
> I'll go get some coffee now and hopefully that is somewhat clear.

I've been thinking about the policy apect of this, and the more I think about
it, the more I wonder if not allowing some sort of common policy in the kernel
is really the right thing to do here. I know thats somewhat blasphemous, but
this isn't really administrative poilcy that we're talking about, at least not
100%. Its more of a behavioral profile that we're trying to enforce. That may
be splitting hairs, but I think theres precidence for the latter. That is to
say, we configure qdiscs to limit traffic flow to certain rates, and configure
policies which drop traffic that violates it (which includes random discard,
which is the antithesis of deterministic policy). I'm not sure I see this as
any different, espcially if we limit its scope. That is to say, why couldn't we
allow the kernel to program a predetermined set of policies that the admin can
set (i.e. offload routing to a hardware cache of X size with an lru
victimization). If other well defined policies make sense, we can add them and
exposes options via iproute2 or some such to set them. For the use case where
such pre-packaged policies don't make sense, we have things like the flow api to
offer users who want to be able to control their hardware in a more fine grained
approach.

Neil