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

[Wei Yang <richardw.yang@linux.intel.com>]

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On Thu, Feb 21, 2019 at 12:46:18PM +0800, Huang, Ying wrote:
>Wei Yang <richardw.yang@linux.intel.com> 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@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 */
>>
>> 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.
>

That's interesting.

I wrote a module to see the exact size of these two structure on my x86_64.

    sizeof(struct device) = 736 = 8 * 92
    sizeof(struct device_private) = 160 = 8 * 20
    sizeof(struct klist_node) = 32 = 8 * 4

Even klist_node has one 4 byte field, c complier would pack the structure to
make it aligned. Which system alignment it would affect?

After the patch, size would change like this:

   struct device          736   ->   704
   struce device_private  160   ->   192

Would this size change affect system?

>Best Regards,
>Huang, Ying
>
>>>
>>>Best Regards,
>>>Rong Chen

-- 
Wei Yang
Help you, Help me