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

[Palmer Dabbelt <palmerdabbelt@google.com>]

On Tue, 04 Feb 2020 12:33:13 PST (-0800), john.fastabend@gmail.com wrote:
> Björn Töpel wrote:
>> On Tue, 28 Jan 2020 at 03:14, Palmer Dabbelt <palmerdabbelt@google.com> 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.

I'd be interested in some sort of benchmark suite for BPF.  Something like
selftests/bpf/benchmarks/ seems like a reasonable place to me.

>
>> 
>> 
>> Björn