linux-kernel - Re: [RFC][PATCH] ftrace,objtool: PC32 based __mcount

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Date:   Tue, 21 Jun 2022 18:27:39 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>, X86 ML <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        "Naveen N . Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [RFC][PATCH] ftrace,objtool: PC32 based __mcount_loc

On Fri, 17 Jun 2022 at 13:40, Mark Rutland <mark.rutland@....com> wrote:
>
> On Fri, Jun 17, 2022 at 01:24:53PM +0200, Peter Zijlstra wrote:
> >
> > Hi,
> >
> > I recently noticed that __mcount_loc is 64bit wide, containing absolute
> > addresses. Since __mcount_loc is a permanent section (not one we drop
> > after boot), this bloats the kernel memory usage for no real purpose.
> >
> > The below patch adds __mcount_loc_32 and objtool support to generate it.
> > This saves, on an x86_64-defconfig + FTRACE, 23975*4 ~= 94K of permanent
> > storage.
>
> We have a similar issue on arm64, which is exacerbated by needing ABS64
> relocations (24 bytes per entry!) adding significant bloat when FTRACE is
> enabled.
>
> It'd be really nice if going forwards compilers could expose an option to
> generate PC32/PREL32 entries directly for this.
>

As opposed to generating absolute references today? Or as opposed to
having to rely on our own tooling?