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From: Alan Kao <alankao@andestech.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: "Palmer Dabbelt" <palmer@sifive.com>,
	"Albert Ou" <albert@sifive.com>,
	"sw-dev@groups.riscv.org" <sw-dev@groups.riscv.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"Greentime Ying-Han Hu(胡英漢)" <greentime@andestech.com>,
	"Zong Zong-Xian Li(李宗憲)" <zong@andestech.com>
Subject: Re: ftrace: Proposal for an Alternative RecordMcount framework
Date: Thu, 1 Mar 2018 10:05:08 +0800	[thread overview]
Message-ID: <20180301020507.GA24550@andestech.com> (raw)
In-Reply-To: <20180227161252.25162d81@vmware.local.home>

On Wed, Feb 28, 2018 at 05:12:52AM +0800, Steven Rostedt wrote:
> On Tue, 27 Feb 2018 18:04:26 +0800
> Alan Kao <alankao@andestech.com> wrote:
>  
> > 1. During the final linking stages, do "objdump vmlinux.o | grep ..." [2]
> 
> Note, doing it at that stage takes the longest time. It makes small
> changes much longer to compile. That said...
>

What if we can have some option to *disable* all the recordmcount.pl lauches
after every .o?  There will be only an oneshot grep for a near-complete
vmlinux binary.

> > 2. Form the output as an ELF objecj
> > 3. Link the object to __mcount_loc_start symbol
> > 4. Done
> > 
> > With the similar reason as the patch [3], we should mark _mcount to be
> > a weak symbol to prevent it from being relaxed later.
> > 
> > We would like to know your opinion and comments on this.
> > Thanks!
> 
> What about just having your arch use recordmcount.c instead? It doesn't
> do any grepping. It is an elf reader and modifier and modifies the .o
> file directly.

Thanks for the hint.  But after a quick scan, it seems that recordmcount.c
processes .o files in a per-file basis, which means that we will still
suffer from the linker relaxation problem.

> 
> Note, I will be rewriting that code in the near future too, to
> consolidate it with objtool.
> 
> -- Steve

Please allow me to state the problem more clearly here.  I hope this helps.

1. locations of mcount are recorded in a per-file basis.
2. to optimize the binary, the linker turns on some aggressive
   options, including relaxation.
3. the optimizations changes the original offset.
4. already recorded mcount call-sites no longer point to their
   real positions.
5. still a linked vmlinux is made.
6. dynamic ftrace breaks the real logic in the kernel space,
   panics happen.

Thanks,
Alan

  reply	other threads:[~2018-03-01  2:08 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-02-27 10:04 ftrace: Proposal for an Alternative RecordMcount framework Alan Kao
2018-02-27 21:12 ` Steven Rostedt
2018-03-01  2:05   ` Alan Kao [this message]
2018-03-07  1:47     ` Alan Kao
2018-03-07  2:00       ` Steven Rostedt

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