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

Andreas Herrmann
2018-07-17T08:50:39Z

On Tue, Jul 17, 2018 at 10:03:41AM +0200, Rafael J. Wysocki wrote:
> 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?

Ondemand governor. Which triggers a lot of PCC related platform calls.
And as Peter noticed already the driver has a severe bottleneck (lock
protecting shared memory used for all CPUs to pass data to/from
platform for PCC calls).

> Does changing it to something like "performance" improve things?

With performance governor above mentioned bottleneck is no issue.

On balance before this commit users could use pcc-cpufreq but had
already suboptimal performance (compared to say intel_pstate driver
which can be used changing BIOS options). Starting with this commit
systems using pcc-cpufreq are unusable with high number of CPUs (top
output above is for system with 120 CPUs).

So should the driver be removed (sooner or later), or this behaviour
be documented somewhere, or just leave it as is.

Andreas