Re: [LKP] Re: [mm/hugetlb] c77c0a8ac4: will-it-scale.per_process_ops 15.9% improvement

[Feng Tang]

Hi Waiman and Michal,

On Tue, Jan 14, 2020 at 09:57:14AM -0500, Waiman Long wrote:
> On 1/14/20 4:12 AM, Michal Hocko wrote:
> > On Tue 14-01-20 16:56:37, kernel test robot wrote:
> >> Greeting,
> >>
> >> FYI, we noticed a 15.9% improvement of will-it-scale.per_process_ops due to commit:
> >>
> >>
> >> commit: c77c0a8ac4c522638a8242fcb9de9496e3cdbb2d ("mm/hugetlb: defer freeing of huge pages if in non-task context")
> >> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
> > This is more than surprising because the patch has only changed the
> > behavior for hugetlb pages freed from the (soft)interrupt context and
> > that should be a very rare event. Does the test really generate a lot of
> > those?
> >
> Yes, I have the same question. I was not expecting to see any
> performance impact.

We have the same question and did some further check.

This is the "pagefault3" test case of will-it-scale, and is  
mmap/get_page/munmap test. The source code is: 
https://github.com/antonblanchard/will-it-scale/blob/master/tests/page_fault3.c 

And its running on LKP does NOT involve any hugetlb actions, as
could be checking HugePages_* in /proc/meminfo.

We also did another check, reverted c77c0a8ac4c5 and simply added
some printk inside free_huge_page(), which can also bring 15%
improvement.

So one possible reason could be the commit changes the cache
alignment of other kernel codes in final bzImage, which happens
to hugely affect this test case.

Thanks,
Feng

> 
> Cheers,
> Longman
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