Re: [LKP] [lkp-robot] [sched/cfs] 625ed2bf04: unixbench.score -7.4% regression

[Peter Zijlstra]

On Mon, Aug 28, 2017 at 01:57:39PM +0800, Huang, Ying wrote:
> kernel test robot <xiaolong.ye@xxxxxxxxx> writes:
> 
> > Greeting,
> >
> > FYI, we noticed a -7.4% regression of unixbench.score due to commit:
> >
> >
> > commit: 625ed2bf049d5a352c1bcca962d6e133454eaaff ("sched/cfs: Make util/load_avg more stable")
> > https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git master
> >
> > in testcase: unixbench
> > on test machine: 88 threads Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz with 64G memory
> > with following parameters:
> >
> > 	runtime: 300s
> > 	nr_task: 100%
> > 	test: spawn
> > 	cpufreq_governor: performance
> >
> > test-description: UnixBench is the original BYTE UNIX benchmark suite aims to test performance of Unix-like system.
> >
> 
> This has been merged by v4.13-rc1, so we checked it again.  If my
> understanding were correct, the patch changes the algorithm to calculate
> the load of CPU, so it influences the load balance behavior for this
> test case.
> 
>       4.73 ±  8%     -31.3%       3.25 ± 10%  sched_debug.cpu.nr_running.max
>       0.95 ±  5%     -29.0%       0.67 ±  4%  sched_debug.cpu.nr_running.stddev
> 
> As above, the effect is that the tasks are distributed into more CPUs,
> that is, system is more balanced.  But this triggered more contention on
> tasklist_lock, so hurt the unixbench score, as below.
> 
>      26.60           -10.6       16.05        perf-profile.calltrace.cycles-pp.intel_idle.cpuidle_enter_state.cpuidle_enter.call_cpuidle.do_idle
>      10.10            +2.4       12.53        perf-profile.calltrace.cycles-pp._raw_write_lock_irq.do_exit.do_group_exit.sys_exit_group.entry_SYSCALL_64_fastpath
>       8.03            +2.6       10.63        perf-profile.calltrace.cycles-pp._raw_write_lock_irq.release_task.wait_consider_task.do_wait.sys_wait4
>      17.98            +5.2       23.14        perf-profile.calltrace.cycles-pp._raw_read_lock.do_wait.sys_wait4.entry_SYSCALL_64_fastpath
>       7.47            +5.9       13.33        perf-profile.calltrace.cycles-pp._raw_write_lock_irq.copy_process._do_fork.sys_clone.do_syscall_64
> 
> 
> The patch makes the tasks distributed more balanced, so I think
> scheduler do better job here.  The problem is that the tasklist_lock
> isn't scalable.  But considering this is only a micro-benchmark which
> specially exercises fork/exit/wait syscall, this may be not a big
> problem in reality.
> 
> So, all in all, I think we can ignore this regression.

Thanks for looking at this!