Re: Commit 554c8aa8ecad causing severe performance degression with pcc-cpufreq

Rafael J. Wysocki
2018-07-17T08:03:41Z

On Tue, Jul 17, 2018 at 9:33 AM, Rafael J. Wysocki <rafael@kernel.org> wrote:
> Hi,
>
> Thanks for your report!
>
> On Tue, Jul 17, 2018 at 8:50 AM, Andreas Herrmann <aherrmann@suse.com> wrote:
>> Hello,
>>
>> I've recently noticed that commit 554c8aa8ecad ("sched: idle: Select
>> idle state before stopping the tick") causes severe performance drop
>> for systems using pcc-cpufreq driver. Depending on the number of CPUs
>> the system might be almost unusable. The OS jitter for 4.17.y and
>> 4.18.-rcx kernels is off the charts, you can even spot it with top
>> command (issued when the system is supposedly idle), e.g.
>>
>> top - 14:44:24 up 2 min, 1 user, load average: 90.11, 38.20, 14.38
>> Tasks: 1199 total, 109 running, 541 sleeping, 0 stopped, 0 zombie
>> %Cpu(s): 1.2 us, 58.7 sy, 0.0 ni, 39.3 id, 0.6 wa, 0.0 hi, 0.3 si, 0.0 st
>> KiB Mem: 13137064+total, 1192168 used, 13017848+free, 2340 buffers
>> KiB Swap: 2104316 total, 0 used, 2104316 free. 522296 cached Mem
>>
>> PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
>> 3373 root 20 0 982024 49916 36120 R 96.691 0.038 0:19.54 kubelet
>> 67 root 20 0 0 0 0 R 78.676 0.000 0:49.36 kworker/9:0
>> 25 root 20 0 0 0 0 R 78.125 0.000 0:49.67 kworker/2:0
>> 182 root 20 0 0 0 0 R 75.735 0.000 1:18.17 kworker/28:0
>> 43 root 20 0 0 0 0 R 75.000 0.000 0:11.56 kworker/5:0
>> 103 root 20 0 0 0 0 R 74.449 0.000 0:46.83 kworker/15:0
>> 334 root 20 0 0 0 0 R 72.978 0.000 1:06.88 kworker/53:0
>> 789 root 20 0 0 0 0 R 69.853 0.000 1:29.50 kworker/38:1
>> 418 root 20 0 0 0 0 R 69.301 0.000 0:41.33 kworker/67:0
>> 779 root 20 0 0 0 0 R 68.934 0.000 1:33.60 kworker/27:1
>> 773 root 20 0 0 0 0 R 68.566 0.000 1:37.91 kworker/22:1
>> 762 root 20 0 0 0 0 R 68.015 0.000 1:41.01 kworker/11:1
>> 769 root 20 0 0 0 0 R 67.647 0.000 1:37.65 kworker/18:1
>> 805 root 20 0 0 0 0 R 67.096 0.000 1:30.96 kworker/54:1
>> 840 root 20 0 0 0 0 R 66.912 0.000 1:23.82 kworker/89:1
>> 812 root 20 0 0 0 0 R 66.728 0.000 1:31.89 kworker/59:1
>> 847 root 20 0 0 0 0 R 66.360 0.000 1:28.40 kworker/96:1
>> 763 root 20 0 0 0 0 R 66.176 0.000 1:42.57 kworker/12:1
>> 772 root 20 0 0 0 0 R 66.176 0.000 1:12.58 kworker/21:1
>> 821 root 20 0 0 0 0 R 66.176 0.000 1:29.62 kworker/69:1
>> 923 root 20 0 0 0 0 R 65.809 0.000 1:44.32 kworker/3:18
>> 1284 root 20 0 0 0 0 R 65.809 0.000 1:23.50 kworker/101:2
>> 61 root 20 0 0 0 0 R 65.625 0.000 1:29.37 kworker/8:0
>> 3531 root 20 0 24384 3768 2356 R 65.625 0.003 0:08.91 top
>> 771 root 20 0 0 0 0 R 65.074 0.000 1:37.90 kworker/20:1
>> 767 root 20 0 0 0 0 R 64.706 0.000 1:38.01 kworker/16:1
>> 764 root 20 0 0 0 0 R 64.522 0.000 1:40.28 kworker/13:1
>> 765 root 20 0 0 0 0 R 64.154 0.000 1:40.13 kworker/14:1
>>
>> When I apply below patch (trying to revert essential parts of commit
>> 554c8aa8ecad) behaviour seems back to normal.
>
> Well, that basically defeats the purpose of the change in commit
> 554c8aa8ecad, so it's not what I'd like to do to fix this problem.
>
> Also it would be good to understand what actually happens.
>
>> I know that pcc-cpufreq driver is not "state-of-the-art" when it comes
>> to cpufreq drivers and you better not use it.
>
> That's exactly right.
>
>> But I wonder whether commit 554c8aa8ecad ("sched: idle: Select idle state before
>> stopping the tick") introduced bad behaviour for other cases as well.
>
> It has been tested quite extensively in that respect, although
> admittedly not with the pcc-cpufreq driver.
>
> Nothing bad related to it has been has been reported so far, FWIW.
>
>> I'll send some performance results to illustrate the issue asap. I've
>> also tried to modify pcc-cpufreq to reduce the amount of frequency
>> changes triggered by this driver but this does not help for kernels
>> where commit 554c8aa8ecad is applied.
>
> Can you replace pcc-cpufreq with a different cpufreq driver on the
> affected systems? If so, do performance numbers look bad after that
> too?

Also, what cpufreq governor do you use with pcc-cpufreq? Does
changing it to something like "performance" improve things?