[PATCH bpf-next v2 0/4] Add ftrace direct call for arm64

[Florent Revest]

On Fri, Oct 21, 2022 at 1:32 PM Masami Hiramatsu <mhiramat at kernel.org> wrote:
> On Mon, 17 Oct 2022 19:55:06 +0200
> Florent Revest <revest at chromium.org> wrote:
> > Mark finished an implementation of his per-callsite-ops and min-args
> > branches (meaning that we can now skip the expensive ftrace's saving
> > of all registers and iteration over all ops if only one is attached)
> > - https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64-ftrace-call-ops-20221017
> >
> > And Masami wrote similar patches to what I had originally done to
> > fprobe in my branch:
> > - https://github.com/mhiramat/linux/commits/kprobes/fprobe-update
> >
> > So I could rebase my previous "bpf on fprobe" branch on top of these:
> > (as before, it's just good enough for benchmarking and to give a
> > general sense of the idea, not for a thorough code review):
> > - https://github.com/FlorentRevest/linux/commits/fprobe-min-args-3
> >
> > And I could run the benchmarks against my rpi4. I have different
> > baseline numbers as Xu so I ran everything again and tried to keep the
> > format the same. "indirect call" refers to my branch I just linked and
> > "direct call" refers to the series this is a reply to (Xu's work)
>
> Thanks for sharing the measurement results. Yes, fprobes/rethook
> implementation is just porting the kretprobes implementation, thus
> it may not be so optimized.
>
> BTW, I remember Wuqiang's patch for kretprobes.
>
> https://lore.kernel.org/all/20210830173324.32507-1-wuqiang.matt@bytedance.com/T/#u

Oh that's a great idea, thanks for pointing it out Masami!

> This is for the scalability fixing, but may possible to improve
> the performance a bit. It is not hard to port to the recent kernel.
> Can you try it too?

I rebased it on my branch
https://github.com/FlorentRevest/linux/commits/fprobe-min-args-3

And I got measurements again. Unfortunately it looks like this does not help :/

New benchmark results: https://paste.debian.net/1257856/
New perf report: https://paste.debian.net/1257859/

The fprobe based approach is still significantly slower than the
direct call approach.

> Anyway, eventually, I would like to remove the current kretprobe
> based implementation and unify fexit hook with function-graph
> tracer. It should make more better perfromance on it.

That makes sense. :) How do you imagine the unified solution ?
Would both the fgraph and fprobe APIs keep existing but under the hood
one would be implemented on the other ? (or would one be gone ?) Would
we replace the rethook freelist with the function graph's per-task
shadow stacks ? (or the other way around ?))

> > Note that I can't really make sense of the perf report with indirect
> > calls. it always reports it spent 12% of the time in
> > rethook_trampoline_handler but I verified with both a WARN in that
> > function and a breakpoint with a debugger, this function does *not*
> > get called when running this "bench trig-fentry" benchmark. Also it
> > wouldn't make sense for fprobe_handler to call it so I'm quite
> > confused why perf would report this call and such a long time spent
> > there. Anyone know what I could be missing here ?

I made slight progress on this. If I put the vmlinux file in the cwd
where I run perf report, the reports no longer contain references to
rethook_trampoline_handler. Instead, they have a few
0xffff800008xxxxxx addresses under fprobe_handler. (like in the
pastebin I just linked)

It's still pretty weird because that range is the vmalloc area on
arm64 and I don't understand why anything under fprobe_handler would
execute there. However, I'm also definitely sure that these 12% are
actually spent getting buffers from the rethook memory pool because if
I replace rethook_try_get and rethook_recycle calls with the usage of
a dummy static bss buffer (for the sake of benchmarking the
"theoretical best case scenario") these weird perf report traces are
gone and the 12% are saved. https://paste.debian.net/1257862/

This is why I would be interested in seeing rethook's memory pool
reimplemented on top of something like
https://lwn.net/Articles/788923/ If we get closer to the performance
of the the theoretical best case scenario where getting a blob of
memory is ~free (and I think it could be the case with a per task
shadow stack like fgraph's), then a bpf on fprobe implementation would
start to approach the performances of a direct called trampoline on
arm64: https://paste.debian.net/1257863/