From 09c60546f04f732d194a171b3a4ccc9ae1e704ba Mon Sep 17 00:00:00 2001 From: Feng Tang Date: Tue, 11 Aug 2020 18:34:13 -0700 Subject: [PATCH] ./Makefile: add debug option to enable function aligned on 32 bytes Recently 0day reported many strange performance changes (regression or improvement), in which there was no obvious relation between the culprit commit and the benchmark at the first look, and it causes people to doubt the test itself is wrong. Upon further check, many of these cases are caused by the change to the alignment of kernel text or data, as whole text/data of kernel are linked together, change in one domain may affect alignments of other domains. gcc has an option '-falign-functions=n' to force text aligned, and with that option enabled, some of those performance changes will be gone, like [1][2][3]. Add this option so that developers and 0day can easily find performance bump caused by text alignment change, as tracking these strange bump is quite time consuming. Though it can't help in other cases like data alignment changes like [4]. Following is some size data for v5.7 kernel built with a RHEL config used in 0day: text data bss dec filename 19738771 13292906 5554236 38585913 vmlinux.noalign 19758591 13297002 5529660 38585253 vmlinux.align32 Raw vmlinux size in bytes: v5.7 v5.7+align32 253950832 254018000 +0.02% Some benchmark data, most of them have no big change: * hackbench: [ -1.8%, +0.5%] * fsmark: [ -3.2%, +3.4%] # ext4/xfs/btrfs * kbuild: [ -2.0%, +0.9%] * will-it-scale: [ -0.5%, +1.8%] # mmap1/pagefault3 * netperf: - TCP_CRR [+16.6%, +97.4%] - TCP_RR [-18.5%, -1.8%] - TCP_STREAM [ -1.1%, +1.9%] [1] https://lore.kernel.org/lkml/20200114085637.GA29297@shao2-debian/ [2] https://lore.kernel.org/lkml/20200330011254.GA14393@feng-iot/ [3] https://lore.kernel.org/lkml/1d98d1f0-fe84-6df7-f5bd-f4cb2cdb7f45@intel.com/ [4] https://lore.kernel.org/lkml/20200205123216.GO12867@shao2-debian/ Signed-off-by: Feng Tang Signed-off-by: Andrew Morton Cc: Masahiro Yamada Cc: Michal Marek Cc: Andi Kleen Cc: Huang Ying Cc: Andy Shevchenko Link: http://lkml.kernel.org/r/1595475001-90945-1-git-send-email-feng.tang@intel.com Signed-off-by: Linus Torvalds --- Makefile | 4 ++++ lib/Kconfig.debug | 11 +++++++++++ 2 files changed, 15 insertions(+) diff --git a/Makefile b/Makefile index 168dd19cad7c..254e80a96b23 100644 --- a/Makefile +++ b/Makefile @@ -893,6 +893,10 @@ KBUILD_CFLAGS += $(CC_FLAGS_SCS) export CC_FLAGS_SCS endif +ifdef CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_32B +KBUILD_CFLAGS += -falign-functions=32 +endif + # arch Makefile may override CC so keep this after arch Makefile is included NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include) diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug index a164785c3b48..ba12a58b2947 100644 --- a/lib/Kconfig.debug +++ b/lib/Kconfig.debug @@ -365,6 +365,17 @@ config SECTION_MISMATCH_WARN_ONLY If unsure, say Y. +config DEBUG_FORCE_FUNCTION_ALIGN_32B + bool "Force all function address 32B aligned" if EXPERT + help + There are cases that a commit from one domain changes the function + address alignment of other domains, and cause magic performance + bump (regression or improvement). Enable this option will help to + verify if the bump is caused by function alignment changes, while + it will slightly increase the kernel size and affect icache usage. + + It is mainly for debug and performance tuning use. + # # Select this config option from the architecture Kconfig, if it # is preferred to always offer frame pointers as a config -- 2.34.1