From: Feng Tang <feng.tang@intel.com>
To: Rong Chen <rong.a.chen@intel.com>,
Linus Torvalds <torvalds@linux-foundation.org>
Cc: Yonghong Song <yhs@fb.com>, Alexei Starovoitov <ast@kernel.org>,
Andrii Nakryiko <andriin@fb.com>, Martin KaFai Lau <kafai@fb.com>,
LKML <linux-kernel@vger.kernel.org>,
"lkp@lists.01.org" <lkp@lists.01.org>
Subject: Re: [LKP] Re: [bpf] af7ec13833: will-it-scale.per_process_ops -2.5% regression
Date: Mon, 13 Jul 2020 17:54:43 +0800 [thread overview]
Message-ID: <20200713095443.GD85993@shbuild999.sh.intel.com> (raw)
In-Reply-To: <1d98d1f0-fe84-6df7-f5bd-f4cb2cdb7f45@intel.com>
On Fri, Jul 03, 2020 at 01:54:39PM +0800, Rong Chen wrote:
> > This commit should not cause regression.
> >
> > Probably the variation of performance is caused by test environment
> > which you may want to investigate further to reduce false alarming.
> > Thanks!
>
> Hi Yonghong,
>
> It's a function align issue, the commit effects the align of functions
> which causes a little regression,
> we force to set -falign-functions=32 in KBUILD_CFLAGS and the regression
> is gone:
>
> diff --git a/Makefile b/Makefile
> index 70def4907036c..9746afa4edc21 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -476,7 +476,7 @@ LINUXINCLUDE牋? := \
> 牋牋牋牋牋牋牋? $(USERINCLUDE)
>
> 燢BUILD_AFLAGS牋 := -D__ASSEMBLY__ -fno-PIE
> -KBUILD_CFLAGS牋 := -Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs \
> +KBUILD_CFLAGS牋 := -Wall -Wundef -falign-functions=32
> -Werror=strict-prototypes -Wno-trigraphs \
> 牋牋牋牋牋牋牋牋牋 -fno-strict-aliasing -fno-common -fshort-wchar
> -fno-PIE \
> 牋牋牋牋牋牋牋牋牋 -Werror=implicit-function-declaration
> -Werror=implicit-int \
> 牋牋牋牋牋牋牋牋牋 -Wno-format-security \
For these strange performance change cases caused by a seemingly
unrelated commit, we have used this function alignment patch to
explain some of them to be caused by re-arrange of text code
alignment [1][2]
So one bold thought is can we merge this option into mainline under
a kernel config option in 'kernel hacking' category, with which both
developers and 0day can filter out some cases to be related with text
alignment change more easily?
[1] [LKP][mm] fd4d9c7d0c: stress-ng.switch.ops_per_sec -30.5% regression
https://lore.kernel.org/lkml/20200330011254.GA14393@feng-iot/
[2] [mm/hugetlb] c77c0a8ac4: will-it-scale.per_process_ops 15.9% improvement
https://lore.kernel.org/lkml/20200114085637.GA29297@shao2-debian/
(this patch is not used in the discussion, but we later used
this patch to confirm it's a text alignment case)
Thanks,
Feng
prev parent reply other threads:[~2020-07-13 9:54 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-28 8:50 [bpf] af7ec13833: will-it-scale.per_process_ops -2.5% regression kernel test robot
2020-06-29 15:10 ` Yonghong Song
2020-07-03 5:54 ` Rong Chen
2020-07-13 9:54 ` Feng Tang [this message]
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