Re: [lkp-robot] [mm] e27be240df: will-it-scale.per_process_ops -27.2% regression

[Aaron Lu <aaron.lu@intel.com>]

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On Tue, May 08, 2018 at 01:26:40PM -0400, Johannes Weiner wrote:
> Hello,
> 
> On Tue, May 08, 2018 at 01:34:51PM +0800, kernel test robot wrote:
> > FYI, we noticed a -27.2% regression of will-it-scale.per_process_ops due to commit:
> > 
> > 
> > commit: e27be240df53f1a20c659168e722b5d9f16cc7f4 ("mm: memcg: make sure memory.events is uptodate when waking pollers")
> > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
> > 
> > in testcase: will-it-scale
> > on test machine: 72 threads Intel(R) Xeon(R) CPU E5-2699 v3 @ 2.30GHz with 128G memory
> > with following parameters:
> > 
> > 	nr_task: 100%
> > 	mode: process
> > 	test: page_fault3
> > 	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
> 
> This is surprising. Do you run these tests in a memory cgroup with a
> limit set? Can you dump that cgroup's memory.events after the run?

There is no cgroup related setup so yes, this is surprising.
But the result is quite stable, I have just confirmed on another
Haswell-EP machine.

perf shows increased cycles spent for lock_page_memcg and
unlock_page_memcg, maybe this can shed some light. Full profile for this
commit and its parent are attached.

I have also attached dmesg for both commits in case they are useful,
please feel free to let me know if you need any other information. We
also collected a ton of other information during the run like
/proc/vmstat, /proc/meminfo, /proc/interrupt etc.

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