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

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Message-ID: <20221017144949.2b9dcdc5@gandalf.local.home>
Date:   Mon, 17 Oct 2022 14:49:49 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Florent Revest <revest@...omium.org>
Cc:     Xu Kuohai <xukuohai@...wei.com>,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Xu Kuohai <xukuohai@...weicloud.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org, Will Deacon <will@...nel.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Zi Shen Lim <zlim.lnx@...il.com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Marc Zyngier <maz@...nel.org>, Guo Ren <guoren@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH bpf-next v2 0/4] Add ftrace direct call for arm64

On Mon, 17 Oct 2022 19:55:06 +0200
Florent Revest <revest@...omium.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