linux-kernel - Re: [PATCH 01/13] objtool: Rewrite hashtable sizing

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Date:   Thu, 10 Jun 2021 20:43:45 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     x86@...nel.org, jpoimboe@...hat.com, jbaron@...mai.com,
        rostedt@...dmis.org, ardb@...nel.org, linux-kernel@...r.kernel.org,
        samitolvanen@...gle.com, ndesaulniers@...gle.com,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH 01/13] objtool: Rewrite hashtable sizing

On Thu, Jun 10, 2021 at 11:14:51AM -0700, Nathan Chancellor wrote:

> This patch as commit 25cf0d8aa2a3 ("objtool: Rewrite hashtable sizing")
> in -tip causes a massive compile time regression with allmodconfig +
> ThinLTO.

Moo... the allyesconfig builds I used it on were much faster, but that
was on regular GCC vmlinux.o after linking.

> Adding Sami because I am not sure why this patch would have much of an impact
> in relation to LTO. https://git.kernel.org/tip/25cf0d8aa2a3 is the patch in
> question.
> 
> If I can provide any further information or help debug, please let me know.
> 
> If you are interested in reproducing this locally, you will need a
> fairly recent LLVM stack (I used the stable release/12.x branch) and to
> cherry-pick commit 976aac5f8829 ("kcsan: Fix debugfs initcall return
> type") to fix an unrelated build failure. My script [2] can build a
> self-contained toolchain fairly quickly if you cannot get one from your
> package manager. A command like below will speed up the build a bit:

Would something like llvm-13 from Debian be good enough?

$ clang-13 --version
Debian clang version 13.0.0-++20210418105309+a0898f0cecc7-1~exp1
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin