Adrián Moreno <[email protected]> writes: > On Thu, Jun 27, 2024 at 09:30:08AM GMT, Aaron Conole wrote: >> 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. >> > > The original purpose of the name was to have the same action for both > userspace and kernel so that, name aside, the semantics > (ACT_XXXX(group=10,cookie=0x123)) remains the same. If we break that, we > risk having userspace and kernel actions differ in ways that makes it > difficult to unify at the xlate/OpenFlow/OVSDB layers. > > But if we can enforce that somehow I guess it's OK.
I think it's less important for that. We do have actions that exist which are datapath specific, so there is precedent. > Thanks. > Adrián _______________________________________________ dev mailing list [email protected] https://mail.openvswitch.org/mailman/listinfo/ovs-dev
