Subject: Re: [PATCH net-next v5 05/10] net: openvswitch: add emit_sample action

From: Aaron Conole <[email-masked]>
Date: Thu, 27 Jun 2024 09:30:08 -0400

Ilya Maximets <[email-masked]> writes:

> On 6/27/24 12:15, Adrián Moreno wrote:
>> On Thu, Jun 27, 2024 at 11:31:41AM GMT, Eelco Chaudron wrote:
>>>
>>>
>>> On 27 Jun 2024, at 11:23, Ilya Maximets wrote:
>>>
>>>> On 6/27/24 11:14, Eelco Chaudron wrote:
>>>>>
>>>>>
>>>>> On 27 Jun 2024, at 10:36, Ilya Maximets wrote:
>>>>>
>>>>>> On 6/27/24 09:52, Adrián Moreno wrote:
>>>>>>> On Thu, Jun 27, 2024 at 09:06:46AM GMT, Eelco Chaudron wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>> On 26 Jun 2024, at 22:34, Adrián Moreno wrote:
>>>>>>>>
>>>>>>>>> On Wed, Jun 26, 2024 at 04:28:17PM GMT, Eelco Chaudron wrote:
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> On 25 Jun 2024, at 22:51, Adrian Moreno wrote:
>>>>>>>>>>
>>>>>>>>>>> Add support for a new action: emit_sample.
>>>>>>>>>>>
>>>>>>>>>>> This action accepts a u32 group id and a variable-length cookie and uses
>>>>>>>>>>> the psample multicast group to make the packet available for
>>>>>>>>>>> observability.
>>>>>>>>>>>
>>>>>>>>>>> The maximum length of the user-defined cookie is set to 16, same as
>>>>>>>>>>> tc_cookie, to discourage using cookies that will not be offloadable.
>>>>>>>>>>
>>>>>>>>>> I’ll add the same comment as I had in the user space part, and that
>>>>>>>>>> is that I feel from an OVS perspective this action should be called
>>>>>>>>>> emit_local() instead of emit_sample() to make it Datapath independent.
>>>>>>>>>> Or quoting the earlier comment:
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> “I’ll start the discussion again on the naming. The name "emit_sample()"
>>>>>>>>>> does not seem appropriate. This function's primary role is to copy the
>>>>>>>>>> packet and send it to a local collector, which varies depending on the
>>>>>>>>>> datapath. For the kernel datapath, this collector is psample, while for
>>>>>>>>>> userspace, it will likely be some kind of probe. This action is distinct
>>>>>>>>>> from the sample() action by design; it is a standalone action that can
>>>>>>>>>> be combined with others.
>>>>>>>>>>
>>>>>>>>>> Furthermore, the action itself does not involve taking a sample; it
>>>>>>>>>> consistently pushes the packet to the local collector. Therefore, I
>>>>>>>>>> suggest renaming "emit_sample()" to "emit_local()". This same goes for
>>>>>>>>>> all the derivative ATTR naming.”
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> This is a blurry semantic area.
>>>>>>>>> IMO, "sample" is the act of extracting (potentially a piece of)
>>>>>>>>> someting, in this case, a packet. It is common to only take some packets
>>>>>>>>> as samples, so this action usually comes with some kind of "rate", but
>>>>>>>>> even if the rate is 1, it's still sampling in this context.
>>>>>>>>>
>>>>>>>>> OTOH, OVS kernel design tries to be super-modular and define small
>>>>>>>>> combinable actions, so the rate or probability generation is done with
>>>>>>>>> another action which is (IMHO unfortunately) named "sample".
>>>>>>>>>
>>>>>>>>> With that interpretation of the term it would actually make more sense
>>>>>>>>> to rename "sample" to something like "random" (of course I'm not
>>>>>>>>> suggestion we do it). "sample" without any nested action that actually
>>>>>>>>> sends the packet somewhere is not sampling, it's just doing something or
>>>>>>>>> not based on a probability. Where as "emit_sample" is sampling even if
>>>>>>>>> it's not nested inside a "sample".
>>>>>>>>
>>>>>>>> You're assuming we are extracting a packet for sampling, but this function
>>>>>>>> can be used for various other purposes. For instance, it could handle the
>>>>>>>> packet outside of the OVS pipeline through an eBPF program (so we are not
>>>>>>>> taking a sample, but continue packet processing outside of the OVS
>>>>>>>> pipeline). Calling it emit_sampling() in such cases could be very
>>>>>>>> confusing.
>>>>>>
>>>>>> We can't change the implementation of the action once it is part of uAPI.
>>>>>> We have to document where users can find these packets and we can't just
>>>>>> change the destination later.
>>>>>
>>>>> I'm not suggesting we change the uAPI implementation, but we could use the
>>>>> emit_xxx() action with an eBPF probe on the action to perform other tasks.
>>>>> This is just an example.
>>>>
>>>> Yeah, but as Adrian said below, you could do that with any action and
>>>> this doesn't change the semantics of the action itself.
>>>
>>> Well this was just an example, what if we have some other need for getting
>>> a packet to userspace through emit_local() other than sampling? The
>>> emit_sample() action naming in this case makes no sense.
>>>
>>>>>>> Well, I guess that would be clearly abusing the action. You could say
>>>>>>> that of anything really. You could hook into skb_consume and continue
>>>>>>> processing the skb but that doesn't change the intended behavior of the
>>>>>>> drop action.
>>>>>>>
>>>>>>> The intended behavior of the action is sampling, as is the intended
>>>>>>> behavior of "psample".
>>>>>>
>>>>>> The original OVS_ACTION_ATTR_SAMPLE "Probabilitically executes actions",
>>>>>> that is it takes some packets from the whole packet stream and executes
>>>>>> actions of them.  Without tying this to observability purposes the name
>>>>>> makes sense as the first definition of the word is "to take a representative
>>>>>> part or a single item from a larger whole or group".
>>>>>>
>>>>>> Now, our new action doesn't have this particular semantic in a way that
>>>>>> it doesn't take a part of a whole packet stream but rather using the
>>>>>> part already taken.  However, it is directly tied to the parent
>>>>>> OVS_ACTION_ATTR_SAMPLE action, since it reports probability of that parent
>>>>>> action.  If there is no parent, then probability is assumed to be 100%,
>>>>>> but that's just a corner case.  The name of a psample module has the
>>>>>> same semantics in its name, it doesn't sample on it's own, but it is
>>>>>> assuming that sampling was performed as it relays the rate of it.
>>>>>>
>>>>>> And since we're directly tied here with both OVS_ACTION_ATTR_SAMPLE and
>>>>>> the psample module, the emit_sample() name makes sense to me.
>>>>>
>>>>> This is the part I don't like. emit_sample() should be treated as a
>>>>> standalone action. While it may have potential dependencies on
>>>>> OVS_ACTION_ATTR_SAMPLE, it should also be perfectly fine to use it
>>>>> independently.
>>>>
>>>> It is fine to use it, we just assume implicit 100% sampling.
>>>
>>> Agreed, but the name does not make sense ;) I do not think we
>>> currently have any actions that explicitly depend on each other
>>> (there might be attributes carried over) and I want to keep it
>>> as such.
>>>
>>>>>>>>> Having said that, I don't have a super strong favor for "emit_sample". I'm
>>>>>>>>> OK with "emit_local" or "emit_packet" or even just "emit".
>>>>>>
>>>>>> The 'local' or 'packet' variants are not descriptive enough on what we're
>>>>>> trying to achieve and do not explain why the probability is attached to
>>>>>> the action, i.e. do not explain the link between this action and the
>>>>>> OVS_ACTION_ATTR_SAMPLE.
>>>>>>
>>>>>> emit_Psample() would be overly specific, I agree, but making the name too
>>>>>> generic will also make it hard to add new actions.  If we use some overly
>>>>>> broad term for this one, we may have to deal with overlapping semantics in
>>>>>> the future.
>>>>>>
>>>>>>>>> I don't think any term will fully satisfy everyone so I hope we can find
>>>>>>>>> a reasonable compromise.
>>>>>>>>
>>>>>>>> My preference would be emit_local() as we hand it off to some local
>>>>>>>> datapath entity.
>>>>>>
>>>>>> What is "local datapath entity" ?  psample module is not part of OVS datapath.
>>>>>> And what is "local" ?  OpenFlow has the OFPP_LOCAL port that is represented
>>>>>> by a bridge port on a datapath level, that will be another source of confusion
>>>>>> as it can be interpreted as sending a packet via a local bridge port.
>>>>>
>>>>> I guess I hinted at a local exit point in the specific netdev/netlink datapath,
>>>>> where exit is to the local host. So maybe we should call it emit_localhost?
>>>>
>>>> For me sending to localhost means sending to a loopback interface or otherwise
>>>> sending the packet to the host networking stack.  And we're not doing that.
>>>
>>> That might be confusing too... Maybe emit_external()?
>> 
>> "External" was the word I used for the original userspace RFC. The
>> rationale being: We're sending the packet to something external from OVS
>> (datapath or userspace). Compared with IPFIX-based observability which
>> where the sample is first processed ("internally") by ovs-vswitchd.
>> 
>> In userspace it kept the sampling/observability meaning because it was
>> part of the Flow_Sample_Collector_Set which is intrinsically an
>> observability thing.
>> 
>> However, in the datapath we loose that meaning and could be confused
>> with some external packet-processing entity. How about "external_observe"
>> or something that somehow keeps that meaning?
>
> This semantics conversation doesn't seem productive as we're going in circles
> around what we already discussed what feels like at least three separate times
> on this and ovs-dev lists.

+1

> I'd say if we can't agree on OVS_ACTION_ATTR_EMIT_SAMPLE, then just call
> it OVS_ACTION_ATTR_SEND_TO_PSAMPLE.  Simple, describes exactly what it does.
> And if we ever want to have "local" sampling for OVS userspace datapath,
> we can create a userspace-only datapath action for it and call it in a way
> that describes what it does, e.g. OVS_ACTION_ATTR_SEND_TO_USDT or whatever.
> Unlike key attributes, we can relatively safely create userspace-only actions
> without consequences for kernel uAPI.  In fact, we have a few such actions.
> And we can choose which one to use based on which one is supported by the
> current datapath.

I'm okay with the emit_sample or with send_to_psample.  There are
probably hundreds of colors to paint this shed, tbh.  We could argue
that it could even be an extension to userspace() instead of a separate
action, or that we could have a generic socket_write(type=psample) type
of action.  But in the end, I don't have a strong feeling either way,
whether it's:

OVS_ACTION_ATTR_EMIT_SAMPLE / emit_sample()
OVS_ACTION_ATTR_SEND_TO_PSAMPLE / psample() or emit_psample()
OVS_ACTION_ATTR_EMIT_EXTERNAL / emit_external()

There aren't really too many differences in them, and it wouldn't bother
me in any case.  I guess a XXX?psample() action does end up being the
clearest since it has 'psample' right in the name and then we can know
right away what it is doing.

> Best regards, Ilya Maximets.