On 11/26/21 7:06 PM, Jakub Kicinski wrote:
On Fri, 26 Nov 2021 13:30:16 +0100 Toke Høiland-Jørgensen wrote:
TBH I wasn't following this thread too closely since I saw Daniel
nacked it already. I do prefer rtnl xstats, I'd just report them
in -s if they are non-zero. But doesn't sound like we have an agreement
whether they should exist or not.
Right, just -s is fine, if we drop the per-channel approach.
I agree that adding them to -s is fine (and that resolves my "no one
will find them" complain as well). If it crowds the output we could also
default to only output'ing a subset, and have the more detailed
statistics hidden behind a verbose switch (or even just in the JSON
output)?
Can we think of an approach which would make cloudflare and cilium
happy? Feels like we're trying to make the slightly hypothetical
admin happy while ignoring objections of very real users.
The initial idea was to only uniform the drivers. But in general
you are right, 10 drivers having something doesn't mean it's
something good.
I don't think it's accurate to call the admin use case "hypothetical".
We're expending a significant effort explaining to people that XDP can
"eat" your packets, and not having any standard statistics makes this
way harder. We should absolutely cater to our "early adopters", but if
we want XDP to see wider adoption, making it "less weird" is critical!
Fair. In all honesty I said that hoping to push for a more flexible
approach hidden entirely in BPF, and not involving driver changes.
Assuming the XDP program has more fine grained stats we should be able
to extract those instead of double-counting. Hence my vague "let's work
with apps" comment.
For example to a person familiar with the workload it'd be useful to
know if program returned XDP_DROP because of configured policy or
failure to parse a packet. I don't think that sort distinction is
achievable at the level of standard stats.
Agree on the additional context. How often have you looked at tc clsact
/dropped/ stats specifically when you debug a more complex BPF program
there?
# tc -s qdisc show clsact dev foo
qdisc clsact ffff: parent ffff:fff1
Sent 6800 bytes 120 pkt (dropped 0, overlimits 0 requeues 0)
backlog 0b 0p requeues 0
Similarly, XDP_PASS counters may be of limited use as well for same reason
(and I think we might not even have a tc counter equivalent for it).
The information required by the admin is higher level. As you say the
primary concern there is "how many packets did XDP eat".
Agree. Above said, for XDP_DROP I would see one use case where you compare
different drivers or bond vs no bond as we did in the past in [0] when
testing against a packet generator (although I don't see bond driver covered
in this series here yet where it aggregates the XDP stats from all bond slave
devs).
On a higher-level wrt "how many packets did XDP eat", it would make sense
to have the stats for successful XDP_{TX,REDIRECT} given these are out
of reach from a BPF prog PoV - we can only count there how many times we
returned with XDP_TX but not whether the pkt /successfully made it/.
In terms of error cases, could we just standardize all drivers on the behavior
of e.g. mlx5e_xdp_handle(), meaning, a failure from XDP_{TX,REDIRECT} will
hit the trace_xdp_exception() and then fallthrough to bump a drop counter
(same as we bump in XDP_DROP then). So the drop counter will account for
program drops but also driver-related drops.
At some later point the trace_xdp_exception() could be extended with an error
code that the driver would propagate (given some of them look quite similar
across drivers, fwiw), and then whoever wants to do further processing with
them can do so via bpftrace or other tooling.
So overall wrt this series: from the lrstats we'd be /dropping/ the pass,
tx_errors, redirect_errors, invalid, aborted counters. And we'd be /keeping/
bytes & packets counters that XDP sees, (driver-)successful tx & redirect
counters as well as drop counter. Also, XDP bytes & packets counters should
not be counted twice wrt ethtool stats.
[0] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9e2ee5c7e7c35d195e2aa0692a7241d47a433d1e
Thanks,
Daniel
