Re: [LKP] [driver core] 570d020012: will-it-scale.per_thread_ops -12.2% regression

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, Feb 21, 2019 at 11:10:49AM +0800, kernel test robot wrote:
> On Tue, Feb 19, 2019 at 01:19:04PM +0100, Greg Kroah-Hartman wrote:
> > On Tue, Feb 19, 2019 at 08:59:45AM +0800, Wei Yang wrote:
> > > On Mon, Feb 18, 2019 at 03:54:42PM +0800, kernel test robot wrote:
> > > >Greeting,
> > > >
> > > >FYI, we noticed a -12.2% regression of will-it-scale.per_thread_ops due to commit:
> > > >
> > > >
> > > >commit: 570d0200123fb4f809aa2f6226e93a458d664d70 ("driver core: move device->knode_class to device_private")
> > > >https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git master
> > > >
> > > 
> > > This is interesting.
> > > 
> > > I didn't expect the move of this field will impact the performance.
> > > 
> > > The reason is struct device is a hotter memory than device->device_private?
> > > 
> > > >in testcase: will-it-scale
> > > >on test machine: 288 threads Knights Mill with 80G memory
> > > >with following parameters:
> > > >
> > > >	nr_task: 100%
> > > >	mode: thread
> > > >	test: unlink2
> > > >	cpufreq_governor: performance
> > > >
> > > >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
> > > >
> > > >In addition to that, the commit also has significant impact on the following tests:
> > > >
> > > >+------------------+---------------------------------------------------------------+
> > > >| testcase: change | will-it-scale: will-it-scale.per_thread_ops -29.9% regression |
> > > >| test machine     | 288 threads Knights Mill with 80G memory                      |
> > > >| test parameters  | cpufreq_governor=performance                                  |
> > > >|                  | mode=thread                                                   |
> > > >|                  | nr_task=100%                                                  |
> > > >|                  | test=signal1                                                  |
> > 
> > Ok, I'm going to blame your testing system, or something here, and not
> > the above patch.
> > 
> > All this test does is call raise(3).  That does not touch the driver
> > core at all.
> > 
> > > >+------------------+---------------------------------------------------------------+
> > > >| testcase: change | will-it-scale: will-it-scale.per_thread_ops -16.5% regression |
> > > >| test machine     | 288 threads Knights Mill with 80G memory                      |
> > > >| test parameters  | cpufreq_governor=performance                                  |
> > > >|                  | mode=thread                                                   |
> > > >|                  | nr_task=100%                                                  |
> > > >|                  | test=open1                                                    |
> > > >+------------------+---------------------------------------------------------------+
> > 
> > Same here, open1 just calls open/close a lot.  No driver core
> > interaction at all there either.
> > 
> > So are you _sure_ this is the offending patch?
> 
> Hi Greg,
> 
> We did an experiment, recovered the layout of struct device. and we
> found the regression is gone. I guess the regession is not from the
> patch but related to the struct layout.
> 
> 
> tests: 1
> testcase/path_params/tbox_group/run: will-it-scale/performance-thread-100%-unlink2/lkp-knm01
> 
> 570d0200123fb4f8  a36dc70b810afe9183de2ea18f  
> ----------------  --------------------------  
>          %stddev      change         %stddev
>              \          |                \  
>     237096              14%     270789        will-it-scale.workload
>        823              14%        939        will-it-scale.per_thread_ops
> 
> 
> tests: 1
> testcase/path_params/tbox_group/run: will-it-scale/performance-thread-100%-signal1/lkp-knm01
> 
> 570d0200123fb4f8  a36dc70b810afe9183de2ea18f  
> ----------------  --------------------------  
>          %stddev      change         %stddev
>              \          |                \  
>      93.51 ±  3%        48%     138.53 ±  3%  will-it-scale.time.user_time
>        186              40%        261        will-it-scale.per_thread_ops
>      53909              40%      75507        will-it-scale.workload
> 
> 
> tests: 1
> testcase/path_params/tbox_group/run: will-it-scale/performance-thread-100%-open1/lkp-knm01
> 
> 570d0200123fb4f8  a36dc70b810afe9183de2ea18f  
> ----------------  --------------------------  
>          %stddev      change         %stddev
>              \          |                \  
>     447722              22%     546258 ± 10%  will-it-scale.time.involuntary_context_switches
>     226995              19%     269751        will-it-scale.workload
>        787              19%        936        will-it-scale.per_thread_ops
> 
> 
> 
> commit a36dc70b810afe9183de2ea18faa4c0939c139ac
> Author: 0day robot <lkp@intel.com>
> Date:   Wed Feb 20 14:21:19 2019 +0800
> 
>     backfile klist_node in struct device for debugging
>     
>     Signed-off-by: 0day robot <lkp@intel.com>
> 
> diff --git a/include/linux/device.h b/include/linux/device.h
> index d0e452fd0bff2..31666cb72b3ba 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -1035,6 +1035,7 @@ struct device {
>  	spinlock_t		devres_lock;
>  	struct list_head	devres_head;
>  
> +	struct klist_node       knode_class_test_by_rongc;
>  	struct class		*class;
>  	const struct attribute_group **groups;	/* optional groups */

While this is fun to worry about alignment and structure size of 'struct
device' I find it odd given that the syscalls and userspace load of
those test programs have nothing to do with 'struct device' at all.

So I can work on fixing up the alignment of struct device, as that's a
nice thing to do for systems with 30k of these in memory, but that
shouldn't affect a workload of a constant string of signal calls.

thanks,

greg k-h