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

[Minchan Kim]

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@xxxxxxxxx> wrote:
> >> 
> >> > Minchan Kim <minchan@xxxxxxxxxx> 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
> >> > pageâ. 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, Iâll 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 Â  3%        -2%       1257 Â  3%        -6%       1207        vmstat.system.cs
>       2702 Â 18%        11%       3002 Â 19%        17%       3156 Â 18%  numa-vmstat.node0.nr_mapped
>      10765 Â 18%        11%      11964 Â 19%        17%      12588 Â 18%  numa-meminfo.node0.Mapped
>       0.00 Â 47%       -40%       0.00 Â 45%       -84%       0.00 Â 42%  mpstat.cpu.soft%
> 
> Thanks,
> Xiaolong

Thanks for the testing!