Messages in this thread | | | Date | Thu, 23 Jun 2022 15:19:59 +0100 | From | Mark Rutland <> | Subject | Re: [RFC][PATCH] ftrace,objtool: PC32 based __mcount_loc |
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On Tue, Jun 21, 2022 at 06:27:39PM +0200, Ard Biesheuvel wrote: > On Fri, 17 Jun 2022 at 13:40, Mark Rutland <mark.rutland@arm.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?
Both; My prefrence would be the compiler had the option to generate these entries as relative at compile-time, without needing a binary post-processing step.
That said, from my PoV this is one of the things that I think it's somewhat reasonable for objtool to do, as it's arguably akin to the build-time sort, and doesn't require deep knowledge of the control flow, etc.
Thanks, Mark.
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