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

[Huang\, Ying]

Wei Yang <richardw.yang@xxxxxxxxxxxxxxx> writes:

> 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
>>
>
> Do you have the comparison between a36dc70b810afe9183de2ea18f and the one
> before 570d020012?
>
>>
>>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@xxxxxxxxx>
>>Date:   Wed Feb 20 14:21:19 2019 +0800
>>
>>    backfile klist_node in struct device for debugging
>>    
>>    Signed-off-by: 0day robot <lkp@xxxxxxxxx>
>>
>>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 */
>
> Hmm... because this is not properly aligned?
>
> struct klist_node {
> 	void			*n_klist;	/* never access directly */
> 	struct list_head	n_node;
> 	struct kref		n_ref;
> };
>
> Except struct kref has one "int" type, others are pointers.
>
> But... I am still confused.

I guess because the size of struct device is changed, it influences some
alignment changes in the system.  Thus influence the benchmark score.

Best Regards,
Huang, Ying

>>
>>Best Regards,
>>Rong Chen