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SubjectRe: [LKP] Re: [sched] bdfcae1140: will-it-scale.per_thread_ops -37.0% regression
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On 10/20/2020 9:14 PM, Mathieu Desnoyers wrote:
> ----- On Oct 19, 2020, at 11:24 PM, Xing Zhengjun zhengjun.xing@linux.intel.com wrote:
>
>> On 10/7/2020 10:50 PM, Mathieu Desnoyers wrote:
>>> ----- On Oct 2, 2020, at 4:33 AM, Rong Chen rong.a.chen@intel.com wrote:
>>>
>>>> Greeting,
>>>>
>>>> FYI, we noticed a -37.0% regression of will-it-scale.per_thread_ops due to
>>>> commit:
>>>>
>>>>
>>>> commit: bdfcae11403e5099769a7c8dc3262e3c4193edef ("[RFC PATCH 2/3] sched:
>>>> membarrier: cover kthread_use_mm (v3)")
>>>> url:
>>>> https://github.com/0day-ci/linux/commits/Mathieu-Desnoyers/Membarrier-updates/20200925-012549
>>>> base: https://git.kernel.org/cgit/linux/kernel/git/tip/tip.git
>>>> 848785df48835eefebe0c4eb5da7690690b0a8b7
>>>>
>>>> in testcase: will-it-scale
>>>> on test machine: 104 threads Skylake with 192G memory
>>>> with following parameters:
>>>>
>>>> nr_task: 50%
>>>> mode: thread
>>>> test: context_switch1
>>>> cpufreq_governor: performance
>>>> ucode: 0x2006906
>>>>
>>>> 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
>>>>
>>>
>>> Hi,
>>>
>>> I would like to report what I suspect is a random thread placement issue in the
>>> context_switch1 test used by the 0day bot when running on a machine with
>>> hyperthread
>>> enabled.
>>>
>>> AFAIU the test code uses hwloc for thread placement which should theoretically
>>> ensure
>>> that each thread is placed on same processing unit, core and numa node between
>>> runs.
>>>
>>> We can find the test code here:
>>>
>>> https://github.com/antonblanchard/will-it-scale/blob/master/tests/context_switch1.c
>>>
>>> And the main file containing thread setup is here:
>>>
>>> https://github.com/antonblanchard/will-it-scale/blob/master/main.c
>>>
>>> AFAIU, the test is started without the "-m" switch, which therefore affinitizes
>>> tasks on cores rather than on processing units (SMT threads).
>>>
>>> When testcase() creates the child thread with new_task(), it basically issues:
>>>
>>> pthread_create(&threads[nr_threads++], NULL, func, arg);
>>>
>>> passing a NULL pthread_attr_t, and not executing any pre_trampoline on the
>>> child.
>>> The pre_trampoline would have issued hwloc_set_thread_cpubind if it were
>>> executed on
>>> the child, but it's not. Therefore, we expect the cpu affinity mask of the
>>> parent to
>>> be copied on clone and used by the child.
>>>
>>> A quick test on a machine with hyperthreading enabled shows that the cpu
>>> affinity mask
>>> for the parent and child has two bits set:
>>>
>>> taskset -p 1868607
>>> pid 1868607's current affinity mask: 10001
>>> taskset -p 1868606
>>> pid 1868606's current affinity mask: 10001
>>>
>>> So AFAIU the placement of the parent and child will be random on either the same
>>> processing unit, or on separate processing units within the same core.
>>>
>>> I suspect this randomness can significantly affect the performance number
>>> between
>>> runs, and trigger unwarranted performance regression warnings.
>>>
>>> Thanks,
>>>
>>> Mathieu
>>>
>> Yes, the randomness may happen in some special cases. But in 0-day, we
>> test multi times (>=3), the report is the average number.
>> For this case, we test 4 times, it is stable, the wave is ± 2%.
>> So I don't think the -37.0% regression is caused by the randomness.
>>
>> 0/stats.json: "will-it-scale.per_thread_ops": 105228,
>> 1/stats.json: "will-it-scale.per_thread_ops": 100443,
>> 2/stats.json: "will-it-scale.per_thread_ops": 98786,
>> 3/stats.json: "will-it-scale.per_thread_ops": 102821,
>>
>> c2daff748f0ea954 bdfcae11403e5099769a7c8dc32
>> ---------------- ---------------------------
>> %stddev %change %stddev
>> \ | \
>> 161714 ± 2% -37.0% 101819 ± 2% will-it-scale.per_thread_ops
>
> Arguing whether this specific instance of the test is indeed a performance
> regression or not is not relevant to this discussion.
>
> What I am pointing out here is that the test needs fixing because it generates
> noise due to a random thread placement configuration. This issue is about whether
> we can trust the results of those tests as kernel maintainers.
>
> So on one hand, you can fix the test. This is simple to do: make sure the thread
> affinity does not allow for this randomness on SMT.
>
> But you seem to argue that the test does not need to be fixed, because the 0day
> infrastructure in which it runs will cover for this randomness. I really doubt
> about this.
>
> If you indeed choose to argue that the test does not need fixing, then here is the
> statistical analysis I am looking for:
>
> - With the 4 runs, what are the odds that the average result for one class significantly
> differs from the other class due to this randomness. It may be small, but it is certainly
> not zero,

If 4 runs are not enough, how many times' run do you think is OK? In
fact, I have re-test it for more than 10 times, the test result is
almost the same.
=========================================================================================
tbox_group/testcase/rootfs/kconfig/compiler/nr_task/mode/test/cpufreq_governor/ucode/debug-setup:

lkp-skl-fpga01/will-it-scale/debian-10.4-x86_64-20200603.cgz/x86_64-rhel-8.3/gcc-9/50%/thread/context_switch1/performance/0x2006906/test2

commit:
c2daff748f0ea954746e8e3465998b1090be7c30
bdfcae11403e5099769a7c8dc3262e3c4193edef

c2daff748f0ea954 bdfcae11403e5099769a7c8dc32
---------------- ---------------------------
%stddev %change %stddev
\ | \
161582 -37.2% 101435 will-it-scale.per_thread_ops
8402288 -37.2% 5274649 will-it-scale.workload


> - Based on those odds, and on the number of performance regression tests performed by 0day
> each year, how frequently does 0day end up spamming kernel developers with random results
> because of this randomness ?
>
> That being said, I would really find more productive that we work together on fixing the
> test rather than justifying why it can stay broken. Let me know if you have specific
> questions on how to fix the test, and I'll be happy to help out.
>
> Thanks,
>
> Mathieu
>
In fact, 0-day just copy the will-it-scale benchmark from the GitHub, if
you think the will-it-scale benchmark has some issues, you can
contribute your idea and help to improve it, later we will update the
will-it-scale benchmark to the new version.
For this test case, if we bind the workload to a specific CPU, then it
will hide the scheduler balance issue. In the real world, we seldom bind
the CPU...

--
Zhengjun Xing

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