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From: Arvind Sankar <nivedita@alum.mit.edu>
To: Andi Kleen <ak@linux.intel.com>
Cc: "Arvind Sankar" <nivedita@alum.mit.edu>,
	"Kees Cook" <keescook@chromium.org>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Will Deacon" <will@kernel.org>,
	"Nick Desaulniers" <ndesaulniers@google.com>,
	"Jian Cai" <jiancai@google.com>,
	"Fāng-ruì Sòng" <maskray@google.com>,
	"Luis Lozano" <llozano@google.com>,
	"Manoj Gupta" <manojgupta@google.com>,
	stable@vger.kernel.org,
	"Catalin Marinas" <catalin.marinas@arm.com>,
	"Mark Rutland" <mark.rutland@arm.com>,
	"Ard Biesheuvel" <ardb@kernel.org>,
	"Peter Collingbourne" <pcc@google.com>,
	"James Morse" <james.morse@arm.com>,
	"Borislav Petkov" <bp@suse.de>, "Ingo Molnar" <mingo@redhat.com>,
	"Russell King" <linux@armlinux.org.uk>,
	"Masahiro Yamada" <masahiroy@kernel.org>,
	"Nathan Chancellor" <natechancellor@gmail.com>,
	"Arnd Bergmann" <arnd@arndb.de>,
	x86@kernel.org, clang-built-linux@googlegroups.com,
	linux-arch@vger.kernel.org, linux-efi@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org,
	"Michal Marek" <michal.lkml@markovi.net>
Subject: Re: [PATCH v5 13/36] vmlinux.lds.h: add PGO and AutoFDO input sections
Date: Tue, 4 Aug 2020 12:06:49 -0400
Message-ID: <20200804160649.GA2409491@rani.riverdale.lan> (raw)
In-Reply-To: <20200804044532.GC1321588@tassilo.jf.intel.com>

On Mon, Aug 03, 2020 at 09:45:32PM -0700, Andi Kleen wrote:
> > Why is that? Both .text and .text.hot have alignment of 2^4 (default
> > function alignment on x86) by default, so it doesn't seem like it should
> > matter for packing density.  Avoiding interspersing cold text among
> 
> You may lose part of a cache line on each unit boundary. Linux has 
> a lot of units, some of them small. All these bytes add up.

Separating out .text.unlikely, which isn't aligned, slightly _reduces_
this loss, but not by much -- just over 1K on a defconfig. More
importantly, it moves cold code out of line (~320k on a defconfig),
giving better code density for the hot code.

For .text and .text.hot, you lose the alignment padding on every
function boundary, not unit boundary, because of the 16-byte alignment.
Whether .text.hot and .text are arranged by translation unit or not
makes no difference.

With *(.text.hot) *(.text) you get HHTT, with *(.text.hot .text) you get
HTHT, but in both cases the individual chunks are already aligned to 16
bytes. If .text.hot _had_ different alignment requirements to .text, the
HHTT should actually give better packing in general, I think.

> 
> It's bad for TLB locality too. Sadly with all the fine grained protection
> changes the 2MB coverage is eroding anyways, but this makes it even worse.
> 

Yes, that could be true for .text.hot, depending on whether the hot
functions are called from all over the kernel (in which case putting
them together ought to be better) or mostly from regular text within the
unit in which they appeared (in which case it would be better together
with that code).

  parent reply index

Thread overview: 70+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-31 23:07 [PATCH v5 00/36] Warn on orphan section placement Kees Cook
2020-07-31 23:07 ` [PATCH v5 01/36] x86/boot/compressed: Move .got.plt entries out of the .got section Kees Cook
2020-08-14 15:23   ` [tip: x86/boot] " tip-bot2 for Ard Biesheuvel
2020-07-31 23:07 ` [PATCH v5 02/36] x86/boot/compressed: Force hidden visibility for all symbol references Kees Cook
2020-08-14 15:23   ` [tip: x86/boot] " tip-bot2 for Ard Biesheuvel
2020-07-31 23:07 ` [PATCH v5 03/36] x86/boot/compressed: Get rid of GOT fixup code Kees Cook
2020-08-14 15:23   ` [tip: x86/boot] " tip-bot2 for Ard Biesheuvel
2020-07-31 23:07 ` [PATCH v5 04/36] x86/boot: Add .text.* to setup.ld Kees Cook
2020-08-14 15:23   ` [tip: x86/boot] " tip-bot2 for Arvind Sankar
2020-07-31 23:07 ` [PATCH v5 05/36] x86/boot: Remove run-time relocations from .head.text code Kees Cook
2020-07-31 23:42   ` Nick Desaulniers
2020-08-14 15:23   ` [tip: x86/boot] " tip-bot2 for Arvind Sankar
2020-07-31 23:07 ` [PATCH v5 06/36] x86/boot: Remove run-time relocations from head_{32,64}.S Kees Cook
2020-08-07 18:12   ` Nick Desaulniers
2020-08-07 20:20     ` Arvind Sankar
2020-08-14 15:23   ` [tip: x86/boot] " tip-bot2 for Arvind Sankar
2020-07-31 23:07 ` [PATCH v5 07/36] x86/boot: Check that there are no run-time relocations Kees Cook
2020-08-14 15:23   ` [tip: x86/boot] " tip-bot2 for Arvind Sankar
2020-07-31 23:07 ` [PATCH v5 08/36] vmlinux.lds.h: Create COMMON_DISCARDS Kees Cook
2020-07-31 23:07 ` [PATCH v5 09/36] vmlinux.lds.h: Add .gnu.version* to COMMON_DISCARDS Kees Cook
2020-07-31 23:07 ` [PATCH v5 10/36] vmlinux.lds.h: Avoid KASAN and KCSAN's unwanted sections Kees Cook
2020-07-31 23:07 ` [PATCH v5 11/36] vmlinux.lds.h: Split ELF_DETAILS from STABS_DEBUG Kees Cook
2020-07-31 23:07 ` [PATCH v5 12/36] vmlinux.lds.h: Add .symtab, .strtab, and .shstrtab to ELF_DETAILS Kees Cook
2020-07-31 23:07 ` [PATCH v5 13/36] vmlinux.lds.h: add PGO and AutoFDO input sections Kees Cook
2020-08-01  3:51   ` Arvind Sankar
2020-08-01  6:18     ` Kees Cook
2020-08-01 17:27       ` Arvind Sankar
2020-08-03 19:05     ` Andi Kleen
2020-08-03 20:15       ` Arvind Sankar
2020-08-04  1:19         ` Fāng-ruì Sòng
2020-08-04  4:45         ` Andi Kleen
2020-08-04  5:32           ` Fāng-ruì Sòng
2020-08-04 16:06           ` Arvind Sankar [this message]
2020-08-21 19:18             ` Kees Cook
2020-07-31 23:07 ` [PATCH v5 14/36] efi/libstub: Disable -mbranch-protection Kees Cook
2020-07-31 23:07 ` [PATCH v5 15/36] arm64/mm: Remove needless section quotes Kees Cook
2020-07-31 23:08 ` [PATCH v5 16/36] arm64/kernel: Remove needless Call Frame Information annotations Kees Cook
2020-07-31 23:08 ` [PATCH v5 17/36] arm64/build: Remove .eh_frame* sections due to unwind tables Kees Cook
2020-07-31 23:08 ` [PATCH v5 18/36] arm64/build: Use common DISCARDS in linker script Kees Cook
2020-07-31 23:08 ` [PATCH v5 19/36] arm64/build: Add missing DWARF sections Kees Cook
2020-07-31 23:08 ` [PATCH v5 20/36] arm64/build: Assert for unwanted sections Kees Cook
2020-07-31 23:08 ` [PATCH v5 21/36] arm64/build: Warn on orphan section placement Kees Cook
2020-07-31 23:08 ` [PATCH v5 22/36] arm/build: Refactor linker script headers Kees Cook
2020-07-31 23:08 ` [PATCH v5 23/36] arm/build: Explicitly keep .ARM.attributes sections Kees Cook
2020-08-03 19:02   ` Nick Desaulniers
2020-08-17 22:06     ` Fangrui Song
2020-07-31 23:08 ` [PATCH v5 24/36] arm/build: Add missing sections Kees Cook
2020-07-31 23:08 ` [PATCH v5 25/36] arm/build: Warn on orphan section placement Kees Cook
2020-07-31 23:08 ` [PATCH v5 26/36] arm/boot: Handle all sections explicitly Kees Cook
2020-07-31 23:08 ` [PATCH v5 27/36] arm/boot: Warn on orphan section placement Kees Cook
2020-07-31 23:08 ` [PATCH v5 28/36] x86/asm: Avoid generating unused kprobe sections Kees Cook
2020-07-31 23:08 ` [PATCH v5 29/36] x86/build: Enforce an empty .got.plt section Kees Cook
2020-08-01  2:12   ` Arvind Sankar
2020-08-01  5:32     ` Kees Cook
2020-08-21 17:49     ` Kees Cook
2020-07-31 23:08 ` [PATCH v5 30/36] x86/build: Assert for unwanted sections Kees Cook
2020-07-31 23:08 ` [PATCH v5 31/36] x86/build: Warn on orphan section placement Kees Cook
2020-07-31 23:08 ` [PATCH v5 32/36] x86/boot/compressed: Reorganize zero-size section asserts Kees Cook
2020-08-01  1:47   ` Arvind Sankar
2020-08-01  2:53     ` Arvind Sankar
2020-08-01  5:36       ` Kees Cook
2020-08-01 17:12         ` Arvind Sankar
2020-08-21 18:24           ` Kees Cook
2020-08-01  5:35     ` Kees Cook
2020-08-01 17:00       ` Arvind Sankar
2020-08-21 18:19     ` Kees Cook
2020-07-31 23:08 ` [PATCH v5 33/36] x86/boot/compressed: Remove, discard, or assert for unwanted sections Kees Cook
2020-07-31 23:08 ` [PATCH v5 34/36] x86/boot/compressed: Add missing debugging sections to output Kees Cook
2020-07-31 23:08 ` [PATCH v5 35/36] x86/boot/compressed: Warn on orphan section placement Kees Cook
2020-07-31 23:08 ` [PATCH v5 36/36] arm/build: Assert for unwanted sections Kees Cook

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