Commit 09c60546 authored by Feng Tang's avatar Feng Tang Committed by Linus Torvalds
Browse files

./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: default avatarFeng Tang <feng.tang@intel.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Michal Marek <michal.lkml@markovi.net>
Cc: Andi Kleen <andi.kleen@intel.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Andy Shevchenko <andriy.shevchenko@intel.com>
Link: http://lkml.kernel.org/r/1595475001-90945-1-git-send-email-feng.tang@intel.com


Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 8043fc14

Makefile

+4 −0
Original line number Diff line number Diff line
@@ -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)

lib/Kconfig.debug

+11 −0
Original line number Diff line number Diff line
@@ -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

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