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Date:   Tue, 04 Feb 2020 12:33:13 -0800
From:   John Fastabend <john.fastabend@gmail.com>
To:     =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn.topel@gmail.com>,
        Palmer Dabbelt <palmerdabbelt@google.com>
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Subject: Re: arm64: bpf: Elide some moves to a0 after calls
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Bj=C3=B6rn T=C3=B6pel 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 anythin=
g.  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 suc=
h that
> > there are some patches that avoid changing any functionality pulled o=
ut 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 actu=
ally 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 cont=
ext 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 f=
unction
> > 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 co=
mpiler,
> > but we're also opening the doors to more complicated BPF JIT optimiza=
tions.
> > 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 som=
e
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 anyth=
ing 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 JI=
T with the
> >   time spent executing the program, but I don't have any feel for wha=
t real BPF
> >   programs look like or have any benchmark suite to run.  Is there so=
mething
> >   out there this should be benchmarked against?  (I'd also like to kn=
ow 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 holdi=
ng
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.

> =

> =

> Bj=C3=B6rn