linux-kernel - Re: [PATCH v5 13/36] vmlinux.lds.h: add PGO and AutoFDO input sections

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Date:   Mon, 3 Aug 2020 12:05:06 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Jian Cai <jiancai@...gle.com>,
        Fāng-ruì Sòng <maskray@...gle.com>,
        Luis Lozano <llozano@...gle.com>,
        Manoj Gupta <manojgupta@...gle.com>, stable@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Peter Collingbourne <pcc@...gle.com>,
        James Morse <james.morse@....com>,
        Borislav Petkov <bp@...e.de>, Ingo Molnar <mingo@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>, x86@...nel.org,
        clang-built-linux@...glegroups.com, linux-arch@...r.kernel.org,
        linux-efi@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org,
        Michal Marek <michal.lkml@...kovi.net>
Subject: Re: [PATCH v5 13/36] vmlinux.lds.h: add PGO and AutoFDO input
 sections

> However, the history of their being together comes from
> 
>   9bebe9e5b0f3 ("kbuild: Fix .text.unlikely placement")
> 
> which seems to indicate there was some problem with having them separated out,
> although I don't quite understand what the issue was from the commit message.

Separating it out is less efficient. Gives worse packing for the hot part
if they are not aligned to 64byte boundaries, which they are usually not. 

It also improves packing of the cold part, but that probably doesn't matter.

-Andi