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From: John Fastabend <john.fastabend@gmail.com>
To: "Björn Töpel" <bjorn.topel@gmail.com>,
	"Palmer Dabbelt" <palmerdabbelt@google.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>,
	Alexei Starovoitov <ast@kernel.org>,
	zlim.lnx@gmail.com, catalin.marinas@arm.com, will@kernel.org,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>, Andrii Nakryiko <andriin@fb.com>,
	Shuah Khan <shuah@kernel.org>, Netdev <netdev@vger.kernel.org>,
	bpf <bpf@vger.kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	LKML <linux-kernel@vger.kernel.org>,
	linux-kselftest@vger.kernel.org,
	clang-built-linux@googlegroups.com, kernel-team@android.com
Subject: Re: arm64: bpf: Elide some moves to a0 after calls
Date: Tue, 04 Feb 2020 12:33:13 -0800
Message-ID: <5e39d509c9edc_63882ad0d49345c08@john-XPS-13-9370.notmuch> (raw)
In-Reply-To: <CAJ+HfNh2csyH2xZtGFXW1zwBEW4+bo_E60PWPydJkB6zZTVx3A@mail.gmail.com>

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.

> 
> 
> Björn



  reply index

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-28  2:11 Palmer Dabbelt
2020-01-28  2:11 ` [PATCH 1/4] selftests/bpf: Elide a check for LLVM versions that can't compile it Palmer Dabbelt
2020-02-11 18:20   ` Nick Desaulniers
2020-01-28  2:11 ` [PATCH 2/4] arm64: bpf: Convert bpf2a64 to a function Palmer Dabbelt
2020-01-28  2:11 ` [PATCH 3/4] arm64: bpf: Split the read and write halves of dst Palmer Dabbelt
2020-01-28  2:11 ` [PATCH 4/4] arm64: bpf: Elide some moves to a0 after calls Palmer Dabbelt
2020-02-04 19:13   ` Björn Töpel
2020-02-11  0:15   ` Alexei Starovoitov
2020-02-04 19:30 ` Björn Töpel
2020-02-04 20:33   ` John Fastabend [this message]
2020-02-18 19:28     ` Palmer Dabbelt

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