On 5 Jun 2024, at 22:23, Adrian Moreno wrote:
> (Was: Add psample support to NXAST_SAMPLE action)
>
> This is the userspace counterpart of the work being done in the kernel
> [1] which is still not merged (hence the RFC state). There, a new
> datapath action is added, called "emit_sample". Being a specific
> action, it promises a more efficient way of making packet samples
> available to observability applications.
>
> From the PoV of ovs-vswitchd, this new action is used to implement
> "local sampling". Local sampling (or lsample for short) is configured
> in a similar way as current per-flow IPFIX sampling, i.e: using the
> Flow_Sample_Collector_Set table and the NXAST_SAMPLE action.
>
> However, instead of sending the sample to an external IPFIX collector
> though the network, the sample is emitted using the new action and
> made available to locally running sample collector.
>
> The specific way emit_sample sends the sample (and the way the local
> collector shall collect it) is datapath-specific.
> Currently, currently only the Linux kernel datapath implements it using
> the psample netlink multicast group.
>
> ~~ Configuration ~~
> Local sampling is configured via a new column in the
> Flow_Sample_Collector_Set (FSCS) table called "local_sample_group".
> Configuring this value is orthogonal to also associating the FSCS
> entry to an entry in the IPFIX table.
>
> Once that entry in the OVSDB is configured, NXAST_SAMPLE actions coming
> from the controller will be translated into the following odp action:
>
> sample(sample={P}%, actions(emit_sample(group={G},cookie={C})))
>
> Where:
> P: Is the sampling probability from NXAST_SAMPLE
> G: Is the group id in the FSCS entry whose "id" matches the one in
> the NXAST_SAMPLE.
> C: Is a 64bit cookie result of concatenating the obs_domain and
> obs_point from the NXAST_SAMPLE, i.e:
> "obs_domain << 32 | obs_point"
> Notes:
> - The parent sample action might be omitted if the probability is
> 100% and there is no IPFIX sampling that requires the use of a
> meter.
>
> ~~ Internal implementation: dpif-lsample ~~
> Internally, a new object called "dpif-lsample" is introduced to track
> the configured local sampling exporters and track statistics based on
> odp flow stats (using xcache).
> It exposes the list of configured exporters and their statistics on a
> new unixctl command called "lsample/show".
>
> ~~ Testing ~~
> The series includes an test utility program than can be executed by
> running "tests/ovstest test-psample". This utility listens
> to packets multicasted by the psample module and prints them (also
> printing the obs_domain and obs_point ids).
>
> ~~ HW Offload ~~
> tc offload is not being introduced in this series as existing sample
> or userspace actions are not currently offloadable. Also some
> improvements need to be implemented in tc for it to be feasible.
>
> [1]
> https://patchwork.kernel.org/project/netdevbpf/cover/[email protected]/
Hi Adrian,
I did not finish reviewing, but I was doing some testing, and I noticed I’m not
getting any warning/error when configuring all of this without Datapath support.
Are you planning to add some user feedback when trying to enable this without
the kernel other than the “Datapath does not support emit_sample” which the
user might not understand relates to this feature?
Cheers,
Eelco
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