Messages in this thread | | | Subject | Re: [LKP] Re: [sched] bdfcae1140: will-it-scale.per_thread_ops -37.0% regression | From | Xing Zhengjun <> | Date | Thu, 22 Oct 2020 09:54:42 +0800 |
| |
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
| |