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

[Rafael J. Wysocki]

On Tue, Jul 17, 2018 at 10:36 AM, Andreas Herrmann <aherrmann@xxxxxxxx> wrote:
> On Tue, Jul 17, 2018 at 09:33:53AM +0200, Rafael J. Wysocki wrote:
>> Hi,
>>
>> Thanks for your report!
>>
>> On Tue, Jul 17, 2018 at 8:50 AM, Andreas Herrmann <aherrmann@xxxxxxxx> 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?
>
> I have no performance numbers yet for other cpufreq drivers on this
> system (checking this commit).  But I'll look it at next.

Thanks!

Generally speaking, pcc-cpufreq is fundamentally not scalable, so the
additional concurrency brought in by the commit in question may have
exposed that weakness if that driver is run on a system with multiple
logical CPUs.