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

From: Wei Yang
Date: Thu Feb 21 2019 - 01:02:55 EST

On Thu, Feb 21, 2019 at 12:46:18PM +0800, Huang, Ying wrote:
>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.
>

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