linux-kernel - Re: [LKP] Re: [mm/hugetlb] c77c0a8ac4: will-it-scale.per_process

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Message-ID: <48f750d3-92b4-98d3-5cab-531304bf9fa1@redhat.com>
Date:   Fri, 17 Jan 2020 09:05:27 -0500
From:   Waiman Long <longman@...hat.com>
To:     Feng Tang <feng.tang@...el.com>, Michal Hocko <mhocko@...nel.org>
Cc:     kernel test robot <rong.a.chen@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Davidlohr Bueso <dbueso@...e.de>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org
Subject: Re: [LKP] Re: [mm/hugetlb] c77c0a8ac4: will-it-scale.per_process_ops
 15.9% improvement

On 1/17/20 1:56 AM, Feng Tang wrote:
> 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
>
That sounds reasonable to me. Thanks for the investigation.

Cheers,
Longman