Re: [kernfs] ea7c5fc39a: stress-ng.stream.ops_per_sec 11827.2% improvement

[kernel test robot <rong.a.chen@intel.com>]

On Sun, Jun 07, 2020 at 09:13:08AM +0800, Ian Kent wrote:
> On Sat, 2020-06-06 at 20:18 +0200, Greg Kroah-Hartman wrote:
> > On Sat, Jun 06, 2020 at 11:52:16PM +0800, kernel test robot wrote:
> > > Greeting,
> > > 
> > > FYI, we noticed a 11827.2% improvement of stress-
> > > ng.stream.ops_per_sec due to commit:
> > > 
> > > 
> > > commit: ea7c5fc39ab005b501e0c7666c29db36321e4f74 ("[PATCH 1/4]
> > > kernfs: switch kernfs to use an rwsem")
> > > url: 
> > > https://github.com/0day-ci/linux/commits/Ian-Kent/kernfs-proposed-locking-and-concurrency-improvement/20200525-134849
> > > 
> > 
> > Seriously?  That's a huge performance increase, and one that feels
> > really odd.  Why would a stress-ng test be touching sysfs?
> 
> That is unusually high even if there's a lot of sysfs or kernfs
> activity and that patch shouldn't improve VFS path walk contention
> very much even if it is present.
> 
> Maybe I've missed something, and the information provided doesn't
> seem to be quite enough to even make a start on it.
> 
> That's going to need some analysis which, for my part, will need to
> wait probably until around rc1 time frame to allow me to get through
> the push down stack (reactive, postponed due to other priorities) of
> jobs I have in order to get back to the fifo queue (longer term tasks,
> of which this is one) list of jobs I need to do as well, ;)
> 
> Please, kernel test robot, more information about this test and what
> it's doing.
> 

Hi Ian,

We increased the timeout of stress-ng from 1s to 32s, and there's only
3% improvement of stress-ng.stream.ops_per_sec:

fefcfc968723caf9  ea7c5fc39ab005b501e0c7666c  testcase/testparams/testbox
----------------  --------------------------  ---------------------------
         %stddev      change         %stddev
             \          |                \  
     10686               3%      11037        stress-ng/cpu-cache-performance-1HDD-100%-32s-ucode=0x500002c/lkp-csl-2sp5
     10686               3%      11037        GEO-MEAN stress-ng.stream.ops_per_sec

It seems the result of stress-ng is inaccurate if test time too
short, we'll increase the test time to avoid unreasonable results,
sorry for the inconvenience.

Best Regards,
Rong Chen