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

[Wei Yang]

On Thu, Feb 21, 2019 at 03:18:22PM +0800, Huang, Ying wrote:
>Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> 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
>>> 
>>> 
>>> 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 */
>>
>> 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.
>
>Hi, Greg,
>
>I don't think this is an issues of struct device.  As you said, struct
>device isn't access much during test.  Struct device may share slab page
>with some other data structures (signal related, or fd related (as in
>some other test cases)), so that the alignment of these data structures
>are affected, so caused the performance regression.
>

I didn't get the point here neither.

slab allocator ask memory from page allocator Page by Page and split the page
into pre-defined size. For example, 128B, 512B... Just as shown in
/proc/slabinfo. 

Per my understanding, each struct device / device_private will sits in its own
aligned space. struct device would sits in 1K slab and struct device_private
would sits in 256B slab, both before and after this patch if I am correct.

Hmm... I am just curious about how this alignment is affected. Maybe I lost
some point?

>Best Regards,
>Huang, Ying
>
>> thanks,
>>
>> greg k-h

-- 
Wei Yang
Help you, Help me