Re: [bpf] af7ec13833: will-it-scale.per_process_ops -2.5% regression

[Rong Chen]

On 6/29/20 11:10 PM, Yonghong Song wrote:
> On 6/28/20 1:50 AM, kernel test robot wrote:
> > Greeting,
> >
> > FYI, we noticed a -2.5% regression of will-it-scale.per_process_ops 
> > due to commit:
> >
> >
> > commit: af7ec13833619e17f03aa73a785a2f871da6d66b ("bpf: Add 
> > bpf_skc_to_tcp6_sock() helper")
> > https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git master
>
> One of previous emails claims that
>     commit: 492e639f0c222784e2e0f121966375f641c61b15 ("bpf: Add 
> bpf_seq_printf and bpf_seq_write helpers")
> is reponsible for 2.5% improvement for will-it-scale.per_process_ops, 
> which I believe is false.
>
> 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    := \
                $(USERINCLUDE)

 KBUILD_AFLAGS   := -D__ASSEMBLY__ -fno-PIE
-KBUILD_CFLAGS   := -Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs \
+KBUILD_CFLAGS   := -Wall -Wundef -falign-functions=32 
-Werror=strict-prototypes -Wno-trigraphs \
                   -fno-strict-aliasing -fno-common -fshort-wchar 
-fno-PIE \
                   -Werror=implicit-function-declaration 
-Werror=implicit-int \
                   -Wno-format-security \


Best Regards,
Rong Chen

> > in testcase: will-it-scale
> > on test machine: 192 threads Intel(R) Xeon(R) Platinum 9242 CPU @ 
> > 2.30GHz with 192G memory
> > with following parameters:
> >
> >     nr_task: 16
> >     mode: process
> >     test: mmap1
> >     cpufreq_governor: performance
> >     ucode: 0x5002f01
> >
> > test-description: Will It Scale takes a testcase and runs it from 1 
> > through to n parallel copies to see if the testcase will scale. It 
> > builds both a process and threads based test in order to see any 
> > differences between the two.
> > test-url: https://github.com/antonblanchard/will-it-scale
> >
> >
> >
> > If you fix the issue, kindly add following tag
> > Reported-by: kernel test robot <rong.a.chen@xxxxxxxxx>
> >
> >
> > Details are as below:
> [...]