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

[Wei Yang]

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.

>
>Best Regards,
>Rong Chen

-- 
Wei Yang
Help you, Help me