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

[Steven Rostedt]

On Mon, 17 Oct 2022 19:55:06 +0200
Florent Revest <revest at chromium.org> wrote:

> 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 ?

The trace shows __bpf_prog_exit, which I'm guessing is tracing the end of
the function. Right?

In which case I believe it must call rethook_trampoline_handler:

 -> fprobe_handler() /* Which could use some "unlikely()" to move disabled
                        paths out of the hot path */

       /* And also calls rethook_try_get () which does a cmpxchg! */

	-> ret_hook()
		-> arch_rethook_prepare()
			Sets regs->lr = arch_rethook_trampoline

On return of the function, it jumps to arch_rethook_trampoline()

  -> arch_rethook_trampoline()
	-> arch_rethook_trampoline_callback()
		-> rethook_trampoline_handler()

So I do not know how it wouldn't trigger the WARNING or breakpoint if you
added it there.

-- Steve