[Cake] issue with Cake and bpf filter

Toke Høiland-Jørgensen toke at toke.dk
Tue Aug 21 17:17:12 EDT 2018

On 21 August 2018 23:06:11 CEST, Pete Heist <pete at heistp.net> wrote:
>> On Aug 21, 2018, at 1:25 PM, Toke Høiland-Jørgensen <toke at toke.dk>
>>>> The next simplest fix is to ignore the flow ID override unless
>>>> in "flows" mode. We can then make valid assumptions about what
>>>> go into the host tables.
>>>> The *right* fix, if we want to maximise functionality, would be to
>>>> pass the result struct by reference into cake_hash(), where it can
>>>> override the *host* IDs (not the flow ID). Users can then choose
>>>> between using the override as a flow ID (by setting "hosts" mode
>>>> instead of "flows"), or retaining the default host-isolation
>>>> with a revised definition of "host".
>>> Ah, making it possible to override both host and flow mode is a
>>> idea! I guess we could use the major/minor distinction in the class
>>> steer this. I'll see if I can't integrate this.
>> So, I implemented this; in the latest commit on github it is again
>> possible to override the flow hashing by setting the class ID with a
>> filter; and the host hash can be overridden by setting the major
>> of the class ID. In my testing the hangs from before are gone, but if
>> anyone else wants to test, please do!
>> I'll write up a description of the filter overrides in the man page,
>> submit the change upstream as well...
>Well that’s good timing for me as I’m wrapping up a small utility/eBPF
>to classify an arbitrary username to either MAC or IP. Here’s the work
>in progress, which is not done yet as flow fairness is still under
>construction, and I haven’t gotten my IPv6 support to pass the rather
>stubborn eBPF verifier: https://github.com/heistp/tc-users

Did you see my classifier? Does subnet-to-flow mapping. https://github.com/tohojo/tc-classifier

Feel free to reuse it in whole or in part...

>With your new code Toke:
>- I so far haven’t seen my VM either crash or suddenly fill its disk
>with logs, which is a bonus. :)


>- With the new major/minor ID distinction, I’d probably use major for
>the user and minor for the flow hash?

Yes. See the latest commit in the tc-adv repo for a man page update explaining it. You can also just set the major ID and let cake do the flow hashing...

>Another thing I haven’t looked into yet is that when fq_codel is the
>qdisc, the eBPF action is only called "once in a while” (start of a new
>flow?) With cake it’s called for every single packet, which is what I
>expected to happen, but very different behavior.

Maybe because fq_codel is not splitting gso packets?

>Lastly, if anyone has time to review even just a little code for what
>is or is not good or idiomatic C, post an issue and I’d appreciate it.
>Yes, I yield to the ‘goto’ proponents when it comes to error handling
>and resource de-allocation. :)

I'll take a look tomorrow :)


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