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

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Date:   Fri, 17 Jun 2022 13:11:42 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Steven Rostedt <rostedt@...dmis.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
Subject: Re: [RFC][PATCH] ftrace,objtool: PC32 based __mcount_loc

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.
> 
> XXX hobbled sorttable for now
> XXX compile tested only
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Nice!

Some nits:

- No need for renaming the section, it can still be called
  '__mcount_loc' regardless?

- No need for a new FTRACE_MCOUNT32_USE_OBJTOOL config option or
  '--mcount32' cmdline option, just change the old ones to be pc32?

- change "32" to "PC32": CC_USING_MCOUNT_LOC_PC32

That will shrink this patch down quite a bit.

-- 
Josh