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

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Message-ID: <YrAjOHDrkXQPLv+6@hirez.programming.kicks-ass.net>
Date:   Mon, 20 Jun 2022 09:35:20 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...nel.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 01:11:42PM -0700, Josh Poimboeuf 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.
> > 
> > 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?

I wanted to avoid mixing them by accidental build funnies, also, it
having a different name makes it easier to check what's what with a
simple readelf.

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

Right, so I did that because of the pending --mcount patches for Power.
If Christophe is on board with that, sure, can do.

> - change "32" to "PC32": CC_USING_MCOUNT_LOC_PC32

Right.