linux-kernel - Re: [RFC PATCH 0/4] Speed booting by sorting ORC unwind tables at build time

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Date:   Thu, 7 Nov 2019 09:46:54 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     shile.zhang@...ux.alibaba.com
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org
Subject: Re: [RFC PATCH 0/4] Speed booting by sorting ORC unwind tables at
 build time

On Thu, Nov 07, 2019 at 10:32:01PM +0800, shile.zhang@...ux.alibaba.com wrote:
> From: Shile Zhang <shile.zhang@...ux.alibaba.com>
> 
> Hi,
> 
> I found the unwind_init taken long time (more than 90ms) in kernel
> booting, mainly spent on sorting the two ORC unwind tables, orc_unwind
> and orc_unwind_ip.
> 
> I also noticed that this issued has reported and discussed last year:
> https://lkml.org/lkml/2018/10/8/342
> But seems no final solution until now, I tried to sort the ORC tables at
> build time, followed the helpful hints from Josh and Ingo in that thread.
> And mainly referred the implementation of 'sortextable' tool:
> https://lore.kernel.org/linux-mips/1334872799-14589-1-git-send-email-ddaney.cavm@gmail.com/
> 
> What I did:
> 
> - Add a Kconfig to control build-time sorting or runtime sorting;
> - Referred 'sortextable', create a similar helper tool 'sortorctable',
>   help to sort the ORC unwind tables at vmlinux link process.
> 
> One potential improvement is to sort the module ORC tables in future.
> 
> Thanks!

Thanks a lot for working on this!

I'd say the new config option isn't needed.  The runtime ORC sorting
logic is unconditionally bad and the code should just be removed.  I saw
recently that it's one of the main offenders for boot time latency.

I also agree with Peter that we should try to reduce the link-time
penalty as much as possible.  But it's a necessary evil to a certain
extent.

-- 
Josh