Re: [LKP] [lkp] [mm] 5c0a85fad9: unixbench.score -6.3% regression

[Minchan Kim]

On Wed, Jun 08, 2016 at 11:58:11AM +0300, Kirill A. Shutemov wrote:
> On Wed, Jun 08, 2016 at 04:41:37PM +0800, Huang, Ying wrote:
> > "Huang, Ying" <ying.huang@xxxxxxxxx> writes:
> > 
> > > "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> writes:
> > >
> > >> On Mon, Jun 06, 2016 at 10:27:24AM +0800, kernel test robot wrote:
> > >>> 
> > >>> FYI, we noticed a -6.3% regression of unixbench.score due to commit:
> > >>> 
> > >>> commit 5c0a85fad949212b3e059692deecdeed74ae7ec7 ("mm: make faultaround produce old ptes")
> > >>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> > >>> 
> > >>> in testcase: unixbench
> > >>> on test machine: lituya: 16 threads Haswell High-end Desktop (i7-5960X 3.0G) with 16G memory
> > >>> with following parameters: cpufreq_governor=performance/nr_task=1/test=shell8
> > >>> 
> > >>> 
> > >>> Details are as below:
> > >>> -------------------------------------------------------------------------------------------------->
> > >>> 
> > >>> 
> > >>> =========================================================================================
> > >>> compiler/cpufreq_governor/kconfig/nr_task/rootfs/tbox_group/test/testcase:
> > >>>   gcc-4.9/performance/x86_64-rhel/1/debian-x86_64-2015-02-07.cgz/lituya/shell8/unixbench
> > >>> 
> > >>> commit: 
> > >>>   4b50bcc7eda4d3cc9e3f2a0aa60e590fedf728c5
> > >>>   5c0a85fad949212b3e059692deecdeed74ae7ec7
> > >>> 
> > >>> 4b50bcc7eda4d3cc 5c0a85fad949212b3e059692de 
> > >>> ---------------- -------------------------- 
> > >>>        fail:runs  %reproduction    fail:runs
> > >>>            |             |             |    
> > >>>           3:4          -75%            :4     kmsg.DHCP/BOOTP:Reply_not_for_us,op[#]xid[#]
> > >>>          %stddev     %change         %stddev
> > >>>              \          |                \  
> > >>>      14321 .  0%      -6.3%      13425 .  0%  unixbench.score
> > >>>    1996897 .  0%      -6.1%    1874635 .  0%  unixbench.time.involuntary_context_switches
> > >>>  1.721e+08 .  0%      -6.2%  1.613e+08 .  0%  unixbench.time.minor_page_faults
> > >>>     758.65 .  0%      -3.0%     735.86 .  0%  unixbench.time.system_time
> > >>>     387.66 .  0%      +5.4%     408.49 .  0%  unixbench.time.user_time
> > >>>    5950278 .  0%      -6.2%    5583456 .  0%  unixbench.time.voluntary_context_switches
> > >>
> > >> That's weird.
> > >>
> > >> I don't understand why the change would reduce number or minor faults.
> > >> It should stay the same on x86-64. Rise of user_time is puzzling too.
> > >
> > > unixbench runs in fixed time mode.  That is, the total time to run
> > > unixbench is fixed, but the work done varies.  So the minor_page_faults
> > > change may reflect only the work done.
> > >
> > >> Hm. Is reproducible? Across reboot?
> > >
> > 
> > And FYI, there is no swap setup for test, all root file system including
> > benchmark files are in tmpfs, so no real page reclaim will be
> > triggered.  But it appears that active file cache reduced after the
> > commit.
> > 
> >     111331 ±  1%     -13.3%      96503 ±  0%  meminfo.Active
> >      27603 ±  1%     -43.9%      15486 ±  0%  meminfo.Active(file)
> > 
> > I think this is the expected behavior of the commit?
> 
> Yes, it's expected.
> 
> After the change faularound would produce old pte. It means there's more
> chance for these pages to be on inactive lru, unless somebody actually
> touch them and flip accessed bit.

Hmm, tmpfs pages should be in anonymous LRU list and VM shouldn't scan
anonymous LRU list on swapless system so I really wonder why active file
LRU is shrunk.