Re: Parallelizing vmlinux BTF encoding. was Re: [RFT] Testing 1.22

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Tue, Jun 15, 2021 at 12:13:55PM -0700, Andrii Nakryiko escreveu:
> On Tue, Jun 15, 2021 at 12:01 PM Arnaldo Carvalho de Melo <acme@kernel.org> wrote:

> > Em Tue, Jun 08, 2021 at 09:59:48AM -0300, Arnaldo Carvalho de Melo escreveu:
> > > Em Mon, Jun 07, 2021 at 05:53:59PM -0700, Andrii Nakryiko escreveu:
> > > > I think it's very fragile and it will be easy to get
> > > > broken/invalid/incomplete BTF. Yonghong already brought up the case

> > > I thought about that as it would be almost like the compiler generating
> > > BTF, but you are right, the vmlinux prep process is a complex beast and
> > > probably it is best to go with the second approach I outlined and you
> > > agreed to be less fragile, so I'll go with that, thanks for your
> > > comments.

> > So, just to write some notes here from what I saw so far:

> > 1. In the LTO cases there are inter-CU references, so the current code
> > combines all CUs into one and we end up not being able to parallelize
> > much. LTO is expensive, so... I'll leave it for later, but yeah, I don't
> > think the current algorithm is ideal, can be improved.
 
> Yeah, let's worry about LTO later.
 
> > 2. The case where there's no inter CU refs, which so far is the most
> > common, seems easier, we create N threads, all sharing the dwarf_loader
> > state and the btf_encoder, as-is now. we can process one CU per thread,
> > and as soon as we finish it, just grab a lock and call
> > btf_encoder__encode_cu() with the just produced CU data structures
> > (tags, types, functions, variables, etc), consume them and delete the
> > CU.
> >
> > So each thread will consume one CU, push it to the 'struct btf' class
> > as-is now and then ask for the next CU, using the dwarf_loader state,
> > still under that lock, then go back to processing dwarf tags, then
> > lock, btf add types, rinse, repeat.
> 
> Hmm... wouldn't keeping a "local" per-thread struct btf and just keep
> appending to it for each processed CU until we run out of CUs be
> simpler?

I thought about this as a logical next step, I would love to have a
'btf__merge_argv(struct btf *btf[]), is there one?

But from what I've read after this first paragraph of yours, lemme try
to rephrase:

1. pahole calls btf_encoder__new(...)

   Creates a single struct btf.

2. dwarf_loader will create N threads, each will call a
dwarf_get_next_cu() that is locked and will return a CU to process, when
it finishes this CU, calls btf_encoder__encode_cu() under an all-threads
lock. Rinse repeat.

Until all the threads have consumed all CUs.

then btf_encoder__encode(), which should be probably renamed to
btf_econder__finish() will call btf__dedup(encoder->btf) and write ELF
or raw file.

My first reaction to your first paragraph was:

Yeah, we can have multiple 'struct btf' instances, one per thread, that
will each contain a subset of DWARF CU's encoded as BTF, and then I have
to merge the per-thread BTF and then dedup. O think my rephrase above is
better, no?

> So each thread does as much as possible locally without any
> locks. And only at the very end we merge everything together and then
> dedup. Or we can even dedup inside each worker before merging final
> btf, that probably would give quite a lot of speed up and some memory
> saving. Would be interesting to experiment with that.
> 
> So I like the idea of a fixed pool of threads (can be customized, and
> I'd default to num_workers == num_cpus), but I think we can and should
> keep them independent for as long as possible.

Sure, this should map the whatever the user passes to -j in the kernel
make command line, if nothing is passed as an argument, then default to
getconf(_NPROCESSORS_ONLN).

There is a nice coincidence here where we probably don't care about -J
anymore and want to deal only with -j (detached btf) that is the same as
what 'make' expects to state how many "jobs" (thread pool size) the user
wants 8-)
 
> Another disadvantage of generating small struct btf and then lock +
> merge is that we don't get as efficient string re-use, we'll churn
> more on string memory allocation. Keeping bigger local struct btfs
> allow for more efficient memory re-use (and probably a tiny bit of CPU
> savings).

I think we're in the same page, the contention for adding the CU to a
single 'struct btf' (amongst all DWARF loading threads) after we just
produced it should be minimal, so we grab all the advantages: locality
of reference, minimal contention as DWARF reading/creating the pahole
internal, neutral, data structures should be higher than adding
types/functions/variables via the libbpf BTF API.

I.e. we can leave paralellizing the BTF _encoding_ for later, what we're
trying to do now is to paralellize the DWARF _loading_, right?
 
> So please consider that, it also seems simpler overall.
 
> > The ordering will be different than what we have now, as some smaller
> > CUs (object files with debug) will be processed faster so will get its
> > btf encoding slot faster, but that, at btf__dedup() time shouldn't make
> > a difference, right?
 
> Right, order doesn't matter.
 
> > I think I'm done with refactoring the btf_encoder code, which should be
> > by now a thin layer on top of the excellent libbpf BTF API, just getting
> > what the previous loader (DWARF) produced and feeding libbpf.
 
> Great.
 
> > I thought about fancy thread pools, etc, researching some pre-existing
> > thing or doing some kthread + workqueue lifting from the kernel but will
> > instead start with the most spartan code, we can improve later.
 
> Agree, simple is good. Really curious how much faster we can get. I
> think anything fancy will give a relatively small improvement. The
> biggest one will come from any parallelization.

And I think that is possible, modulo elfutils libraries saying no, I
hope that will not be the case.

- Arnaldo