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

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Date:   Thu, 23 Jun 2022 15:21:30 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Josh Poimboeuf <jpoimboe@...hat.com>,
        christophe.leroy@...roup.eu, naveen.n.rao@...ux.vnet.ibm.com,
        mbenes@...e.cz, Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [RFC][PATCH] ftrace,objtool: PC32 based __mcount_loc

On Wed, Jun 22, 2022 at 10:54:36AM -0400, Steven Rostedt wrote:
> On Fri, 17 Jun 2022 12:40:00 +0100
> Mark Rutland <mark.rutland@....com> wrote:
> 
> > 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.
> 
> I have patches that bring down the size quite a bit. The mcount loc is
> read into the dyn_rec, which has two longs (the second long is the
> flags that only use 32 bits, but is a long to make it aligned, as a 64
> bit word followed by a 32bit word just added 32 bits of padding to make
> it an array).
> 
> The patches make it into two ints (which bring down the size for 64 bit
> machines). The lists are broken up into blocks, and what I do is put
> the top 32 bits of a word into the top of the block, and make sure that
> they are the same among all the entries in the block.
> 
> I guess its time to bring this back alive.

I don't think that helps? I'm on about the size of the kernel "Image" file, not
the runtime memory footprint.

... unless you mean doing that at compiler time?

Mark.