Re: [sched/numa] 0fb3978b0a: stress-ng.fstat.ops_per_sec -18.9% regression

[Huang, Ying]

Hi, Oliver,

Thanks for report.

I still cannot connect the regression with the patch yet.  To double
check, I have run test again with "sched_verbose" kernel command line,
and verified that the sched_domain isn't changed at all with the patch.

kernel test robot <oliver.sang@xxxxxxxxx> writes:
>       0.11   6%      +0.1        0.16   4%  perf-profile.self.cycles-pp.update_rq_clock
>       0.00            +0.1        0.06   6%  perf-profile.self.cycles-pp.memset_erms
>       0.00            +0.1        0.07   5%  perf-profile.self.cycles-pp.get_pid_task
>       0.06   7%      +0.1        0.17   6%  perf-profile.self.cycles-pp.select_task_rq_fair
>       0.54   5%      +0.1        0.68        perf-profile.self.cycles-pp.lockref_put_return
>       4.26            +1.1        5.33        perf-profile.self.cycles-pp.common_perm_cond
>      15.45            +4.9       20.37        perf-profile.self.cycles-pp.lockref_put_or_lock
>      20.12            +6.7       26.82        perf-profile.self.cycles-pp.lockref_get_not_dead

>From the perf-profile above, the most visible change is more cycles in
lockref_get_not_dead(), which will loop with cmpxchg on
dentry->d_lockref.  So this appears to be related to the memory layout.
I will try to debug that.

Because stress-ng is a weird "benchmark" although it's a very good
functionality test, and I cannot connect the patch with the test case
and performance metrics collected.  I think this regression should be a
low priority one which shouldn't prevent the merging etc.  But I will
continue to investigate the regression to try to root cause it.

Best Regards,
Huang, Ying