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

[Huang, Ying]

Hi, All,

"Huang, Ying" <ying.huang@xxxxxxxxx> writes:

> 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.

Done more investigation for this.  It turns out the sched_domain has
been changed after commit 0fb3978b0a, although it's not shown in default
sched_verbose output.  sd->imb_numa_nr of level "NUMA" has been changed
from 24 to 12 after the commit.  So the following debug patch restore
the performance.