Ilya Maximets <[email protected]> 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. _______________________________________________ dev mailing list [email protected] https://mail.openvswitch.org/mailman/listinfo/ovs-dev
