Re: [lkp-robot] [mm] 7674270022: will-it-scale.per_process_ops -19.3% regression

Nadav Amit
2017-08-10T04:14:57Z

Minchan Kim <minchan@kernel.org> wrote:

> On Wed, Aug 09, 2017 at 10:59:02AM +0800, Ye Xiaolong wrote:
>> On 08/08, Minchan Kim wrote:
>>> On Mon, Aug 07, 2017 at 10:51:00PM -0700, Nadav Amit wrote:
>>>> Nadav Amit <nadav.amit@gmail.com> wrote:
>>>>
>>>>> Minchan Kim <minchan@kernel.org> wrote:
>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> On Tue, Aug 08, 2017 at 09:19:23AM +0800, kernel test robot wrote:
>>>>>>> Greeting,
>>>>>>>
>>>>>>> FYI, we noticed a -19.3% regression of will-it-scale.per_process_ops due to commit:
>>>>>>>
>>>>>>>
>>>>>>> commit: 76742700225cad9df49f05399381ac3f1ec3dc60 ("mm: fix MADV_[FREE|DONTNEED] TLB flush miss problem")
>>>>>>> url: https://github.com/0day-ci/linux/commits/Nadav-Amit/mm-migrate-prevent-racy-access-to-tlb_flush_pending/20170802-205715
>>>>>>>
>>>>>>>
>>>>>>> in testcase: will-it-scale
>>>>>>> on test machine: 88 threads Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz with 64G memory
>>>>>>> with following parameters:
>>>>>>>
>>>>>>> nr_task: 16
>>>>>>> mode: process
>>>>>>> test: brk1
>>>>>>> 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
>>>>>>
>>>>>> Thanks for the report.
>>>>>> Could you explain what kinds of workload you are testing?
>>>>>>
>>>>>> Does it calls frequently madvise(MADV_DONTNEED) in parallel on multiple
>>>>>> threads?
>>>>>
>>>>> According to the description it is "testcase:brk increase/decrease of one
>>>>> page”. According to the mode it spawns multiple processes, not threads.
>>>>>
>>>>> Since a single page is unmapped each time, and the iTLB-loads increase
>>>>> dramatically, I would suspect that for some reason a full TLB flush is
>>>>> caused during do_munmap().
>>>>>
>>>>> If I find some free time, I’ll try to profile the workload - but feel free
>>>>> to beat me to it.
>>>>
>>>> The root-cause appears to be that tlb_finish_mmu() does not call
>>>> dec_tlb_flush_pending() - as it should. Any chance you can take care of it?
>>>
>>> Oops, but with second looking, it seems it's not my fault. ;-)
>>> https://marc.info/?l=linux-mm&m=150156699114088&w=2
>>>
>>> Anyway, thanks for the pointing out.
>>> xiaolong.ye, could you retest with this fix?
>>
>> I've queued tests for 5 times and results show this patch (e8f682574e4 "mm:
>> decrease tlb flush pending count in tlb_finish_mmu") does help recover the
>> performance back.
>>
>> 378005bdbac0a2ec 76742700225cad9df49f053993 e8f682574e45b6406dadfffeb4
>> ---------------- -------------------------- --------------------------
>> %stddev change %stddev change %stddev
>> \ | \ | \
>> 3405093 -19% 2747088 -2% 3348752 will-it-scale.per_process_ops
>> 1280 ± 3% -2% 1257 ± 3% -6% 1207 vmstat.system.cs
>> 2702 ± 18% 11% 3002 ± 19% 17% 3156 ± 18% numa-vmstat.node0.nr_mapped
>> 10765 ± 18% 11% 11964 ± 19% 17% 12588 ± 18% numa-meminfo.node0.Mapped
>> 0.00 ± 47% -40% 0.00 ± 45% -84% 0.00 ± 42% mpstat.cpu.soft%
>>
>> Thanks,
>> Xiaolong
>
> Thanks for the testing!

Sorry again for screwing your patch, Minchan.