linux-kernel - Re: [BUG] tracing: dynamic ftrace selftest detected failures

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Message-Id: <20240821070539.981b42e5f3b939c5ce5e3a71@kernel.org>
Date: Wed, 21 Aug 2024 07:05:39 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Sami Tolvanen <samitolvanen@...gle.com>
Cc: Mark Rutland <mark.rutland@....com>, Masami Hiramatsu
 <mhiramat@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, Linux Trace
 Kernel <linux-trace-kernel@...r.kernel.org>, linux-kernel@...r.kernel.org,
 clang-built-linux <llvm@...ts.linux.dev>, Nathan Chancellor
 <nathan@...nel.org>
Subject: Re: [BUG] tracing: dynamic ftrace selftest detected failures

On Tue, 20 Aug 2024 08:10:42 -0700
Sami Tolvanen <samitolvanen@...gle.com> wrote:

> On Tue, Aug 20, 2024 at 3:48 AM Mark Rutland <mark.rutland@....com> wrote:
> >
> > On Tue, Aug 20, 2024 at 10:03:30AM +0900, Masami Hiramatsu wrote:
> > > On Mon, 19 Aug 2024 12:02:44 -0400
> > > Steven Rostedt <rostedt@...dmis.org> wrote:
> > >
> > > > On Tue, 20 Aug 2024 00:56:49 +0900
> > > > Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:
> > > > >
> > > > > >
> > > > > > We may need to add "noinline" or something to make sure those functions
> > > > > > don't get inlined for LTO.
> > > > >
> > > > > Yeah, we need such option at least for function call test.
> > > >
> > > > Could you add the noinline, and if it fixes the issue send a patch?
> > >
> > > I found the target function already has "noinline". I tried to add noinline
> > > to the testing function (callsite), but it also did not work.
> > > I think "noinline" is for the compiler, but LTO is done by the linker.
> >
> > If LTO is breaking noinline, then that has much larger implications for
> > noinstr code and similar, and means that LTO is unsound...
> 
> The noinline attribute is preserved in LLVM IR, so it should continue
> to work with LTO. Which function are we talking about here? Are you
> sure the function was inlined instead of being dropped completely?
> Does marking the function __used help?

We are talking about trace_selftest_startup_dynamic_tracing() in
kernel/trace/trace_selftest.c. The callee is func() which is actually
DYN_FTRACE_TEST_NAME() in kernel/trace/trace_selftest_dynamic.c.
That function passed as pointer (but the compiler can embed it by constant
propagation.)

Does the noinline attribute prevent embedding callsite too? I mean

extern callee()

noinline callee()
{
...
}

caller()
{
	callee() // (*)
}

In this case, does noinline prevent LTO to embed the callee at the callsite(*)
or prevent LTO remove the callee() symbol?

Thank you,

> 
> Sami
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>