On Mon, Sep 20, 2021 at 08:11:10AM -0700, Dave Marchevsky wrote:

The verifier currently logs some useful statistics in
print_verification_stats. Although the text log is an effective feedback
tool for an engineer iterating on a single application, it would also be
useful to enable tracking these stats in a more structured form for
fleetwide or historical analysis, which this patchset attempts to do.

A concrete motivating usecase which came up in recent weeks:

A team owns a complex BPF program, with various folks extending its
functionality over the years. An engineer tries to make a relatively
simple addition but encounters "BPF program is too large. Processed
1000001 insn". 

Their changes bumped the processed insns from 700k to over the limit and
there's no obvious way to simplify. They must now consider a large
refactor in order to incorporate the new feature. What if there was some
previous change which bumped processed insns from 200k->700k which
_could_ be modified to stress verifier less? Tracking historical
verifier stats for each version of the program over the years would
reduce manual work necessary to find such a change.


Although parsing the text log could work for this scenario, a solution
that's resilient to log format and other verifier changes would be
preferable.

This patchset adds a bpf_prog_verif_stats struct - containing the same
data logged by print_verification_stats - which can be retrieved as part
of bpf_prog_info. Looking for general feedback on approach and a few
specific areas before fleshing it out further:

* None of my usecases require storing verif_stats for the lifetime of a
  loaded prog, but adding to bpf_prog_aux felt more correct than trying
  to pass verif_stats back as part of BPF_PROG_LOAD
* The verif_stats are probably not generally useful enough to warrant
  inclusion in fdinfo, but hoping to get confirmation before removing
  that change in patch 1
* processed_insn, verification_time, and total_states are immediately
  useful for me, rest were added for parity with
	print_verification_stats. Can remove.
* Perhaps a version field would be useful in verif_stats in case future
  verifier changes make some current stats meaningless
* Note: stack_depth stat was intentionally skipped to keep patch 1
  simple. Will add if approach looks good.

Sorry for the delay. LPC consumes a lot of mental energy :)

I see the value of exposing some of the verification stats as prog_info.
Let's look at the list:
struct bpf_prog_verif_stats {
       __u64 verification_time;
       __u32 insn_processed;
       __u32 max_states_per_insn;
       __u32 total_states;
       __u32 peak_states;
       __u32 longest_mark_read_walk;
};
verification_time is non deterministic. It varies with frequency
and run-to-run. I don't see how alerting tools can use it.

insn_processed is indeed the main verification metric.
By now it's well known and understood.

max_states_per_insn, total_states, etc were the metrics I've studied
carefully with pruning, back tracking and pretty much every significant
change I did or reiviewed in the verifier. They're useful to humans
and developers, but I don't see how alerting tools will use them.

So it feels to me that insn_processed alone will be enough to address the
monitoring goal.
It can be exposed to fd_info and printed by bpftool.
If/when it changes with some future verifier algorithm we should be able
to approximate it.