Re: arm64: bpf: Elide some moves to a0 after calls

[John Fastabend]

BjÃrn TÃpel wrote:
> On Tue, 28 Jan 2020 at 03:14, Palmer Dabbelt <palmerdabbelt@xxxxxxxxxx> wrote:
> >
> > There's four patches here, but only one of them actually does anything.  The
> > first patch fixes a BPF selftests build failure on my machine and has already
> > been sent to the list separately.  The next three are just staged such that
> > there are some patches that avoid changing any functionality pulled out from
> > the whole point of those refactorings, with two cleanups and then the idea.
> >
> > Maybe this is an odd thing to say in a cover letter, but I'm not actually sure
> > this patch set is a good idea.  The issue of extra moves after calls came up as
> > I was reviewing some unrelated performance optimizations to the RISC-V BPF JIT.
> > I figured I'd take a whack at performing the optimization in the context of the
> > arm64 port just to get a breath of fresh air, and I'm not convinced I like the
> > results.
> >
> > That said, I think I would accept something like this for the RISC-V port
> > because we're already doing a multi-pass optimization for shrinking function
> > addresses so it's not as much extra complexity over there.  If we do that we
> > should probably start puling some of this code into the shared BPF compiler,
> > but we're also opening the doors to more complicated BPF JIT optimizations.
> > Given that the BPF JIT appears to have been designed explicitly to be
> > simple/fast as opposed to perform complex optimization, I'm not sure this is a
> > sane way to move forward.
> >
> 
> Obviously I can only speak for myself and the RISC-V JIT, but given
> that we already have opened the door for more advanced translations
> (branch relaxation e.g.), I think that this makes sense. At the same
> time we don't want to go all JVM on the JITs. :-P

I'm not against it although if we start to go this route I would want some
way to quantify how we are increasing/descreasing load times.

> 
> > I figured I'd send the patch set out as more of a question than anything else.
> > Specifically:
> >
> > * How should I go about measuring the performance of these sort of
> >   optimizations?  I'd like to balance the time it takes to run the JIT with the
> >   time spent executing the program, but I don't have any feel for what real BPF
> >   programs look like or have any benchmark suite to run.  Is there something
> >   out there this should be benchmarked against?  (I'd also like to know that to
> >   run those benchmarks on the RISC-V port.)
> 
> If you run the selftests 'test_progs' with -v it'll measure/print the
> execution time of the programs. I'd say *most* BPF program invokes a
> helper (via call). It would be interesting to see, for say the
> selftests, how often the optimization can be performed.
> 
> > * Is this the sort of thing that makes sense in a BPF JIT?  I guess I've just
> >   realized I turned "review this patch" into a way bigger rabbit hole than I
> >   really want to go down...
> >
> 
> I'd say 'yes'. My hunch, and the workloads I've seen, BPF programs are
> usually loaded, and then resident for a long time. So, the JIT time is
> not super critical. The FB/Cilium folks can definitely provide a
> better sample point, than my hunch. ;-)

In our case the JIT time can be relevant because we are effectively holding
up a kubernetes pod load waiting for programs to load. However, we can
probably work-around it by doing more aggressive dynamic linking now that
this is starting to land.

It would be interesting to have a test to measure load time in selftests
or selftests/benchmark/ perhaps. We have some of these out of tree we
could push in I think if there is interest.

> 
> 
> BjÃrn