Re: [RFC][PATCH] ftrace,objtool: PC32 based __mcount_loc

[Ard Biesheuvel]

On Fri, 17 Jun 2022 at 13:40, Mark Rutland <mark.rutland@xxxxxxx> 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?