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

Nadav Amit
2017-08-08T04:23:40Z

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.

Nadav