Re: [PATCH v9] pgo: add clang's Profile Guided Optimization infrastructure

On Mon, Jun 14, 2021 at 04:16:16PM +0200, 'Marco Elver' via Clang Built Linux wrote:

On Mon, 14 Jun 2021 at 12:45, Peter Zijlstra [off-list ref] wrote:
[...]

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I've also been led to believe that the KCOV data format is not in fact
dependent on which toolchain is used.

Correct, we use KCOV with both gcc and clang. Both gcc and clang emit
the same instrumentation for -fsanitize-coverage. Thus, the user-space
portion and interface is indeed identical:
https://www.kernel.org/doc/html/latest/dev-tools/kcov.html

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I'm thinking it might be about time to build _one_ infrastructure for
that and define a kernel arc format and call it a day.

That may be nice, but it's a rather large request.

Given GCOV just died, perhaps you can look at what KCOV does and see if
that can be extended to do as you want. KCOV is actively used and
we actually tripped over all the fun little noinstr bugs at the time.

There might be a subtle mismatch between coverage instrumentation for
testing/fuzzing and for profiling. (Disclaimer: I'm not too familiar
with Clang-PGO's requirements.) For example, while for testing/fuzzing
we may only require information if a code-path has been visited, for
profiling the "hotness" might be of interest. Therefore, the
user-space exported data format can make several trade-offs in
complexity.

This has been my primary take-away: given that Clang's PGO is different
enough from the other things and provides more specific/actionable
results, I think it's justified to exist on its own separate from the
other parts.

In theory, I imagine there's a limit to how generic one could make
profiling information, because one compiler's optimizations are not
another compiler's optimizations. On the other hand, it may be doable
to collect unified profiling information for common stuff, but I guess
there's little motivation for figuring out the common ground given the
producer and consumer of the PGO data is the same compiler by design
(unlike coverage info for testing/fuzzing).

Therefore, if KCOV's exposed information does not match PGO's
requirements today, I'm not sure what realistically can be done
without turning KCOV into a monster. Because KCOV is optimized for
testing/fuzzing coverage, and I'm not sure how complex we can or want
to make it to cater to a new use-case.

My intuition is that the simpler design is to have 2 subsystems for
instrumentation-based coverage collection: one for testing/fuzzing,
and the other for profiling.

Alas, there's the problem of GCOV, which should be replaceable by KCOV
for most use cases. But it would be good to hear from a GCOV user if
there are some.

But as we learned GCOV is broken on x86 now, I see these options:

1. Remove GCOV, make KCOV the de-facto test-coverage collection
subsystem. Introduce PGO-instrumentation subsystem for profile
collection only, and make it _very_ clear that KCOV != PGO data as
hinted above. A pre-requisite is that compiler-support for PGO
instrumentation adds selective instrumentation support, likely just
making attribute no_instrument_function do the right thing.

Right. I can't speak to GCOV, but KCOV certainly isn't PGO.

2. Like (1) but also keep GCOV, given proper support for attribute
no_instrument_function would probably fix it (?).

3. Keep GCOV (and KCOV of course). Somehow extract PGO profiles from KCOV.

4. Somehow extract PGO profiles from GCOV, or modify kernel/gcov to do so.

If there *is* a way to "combine" these, I don't think it makes sense
to do it now. PGO has users (and is expanding[1]), and trying to
optimize the design before even landing the first version seems like a
needless obstruction, and to likely not address currently undiscovered
requirements.

So, AFAICT, the original blocking issue ("PGO does not respect noinstr")
is not actually an issue (noinstr contains notrace, which IS respected
by PGO[2]), I think this is fine to move forward.

-Kees

[1] https://lore.kernel.org/lkml/20210612032425.11425-1-jarmo.tiitto@gmail.com/ (local)
[2] https://lore.kernel.org/lkml/CAGG=3QVHkkJ236mCJ8Jt_6JtgYtWHV9b4aVXnoj6ypc7GOnc0A@mail.gmail.com/ (local)

-- 
Kees Cook