linux-kernel - Re: [PATCH v5] x86: Avoid relocation information in final vmlinux

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Message-ID: <e8a36925-2965-1dae-da01-5d06ba6747c8@intel.com>
Date:   Mon, 20 Mar 2023 13:12:07 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Petr Pavlu <petr.pavlu@...e.com>, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com
Cc:     nathan@...nel.org, ndesaulniers@...gle.com, trix@...hat.com,
        corbet@....net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5] x86: Avoid relocation information in final vmlinux

On 3/20/23 05:10, Petr Pavlu wrote:
> The issue is then that the collected vmlinux file and hence distribution
> packages end up unnecessarily large because of this extra data. The
> following is a size comparison of vmlinux v6.0 with and without the
> relocation information:
> | Configuration      | With relocs | Stripped relocs |
> | x86_64_defconfig   |       70 MB |           43 MB |
> | +CONFIG_DEBUG_INFO |      818 MB |          367 MB |
> 
> Optimize a resulting vmlinux by adding a postlink step that splits the
> relocation information into vmlinux.relocs and then strips it from the
> vmlinux binary.

When I saw that this adds a postlink step, I read that as, "adds another
step to the unbearably slow single-threaded part of kernel builds". :)

But, here's one data point that made me feel a lot better.  Using a
random .config:

> https://sr71.net/~dave/intel/config-reloctest

the builds get a _bit_ slower, going from 37.0s->37.7s.  This is pretty
arbitrary of course, using my compiler on my hardware, so YMMV.  But,
for me, this seems like a reasonable tradeoff given the space savings.

I'd be curious what other people are seeing.