Commit 554c8aa8ecad causing severe performance degression with pcc-cpufreq

Andreas Herrmann
2018-07-17T06:50:48Z

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.

I know that pcc-cpufreq driver is not "state-of-the-art" when it comes
to cpufreq drivers and you better not use it. But I wonder whether
commit 554c8aa8ecad ("sched: idle: Select idle state before stopping
the tick") introduced bad behaviour for other cases as well.

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.

Andreas

---

diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index 1a3e9bddd17b..377a62ec475c 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -185,18 +185,13 @@ static void cpuidle_idle_call(void)
} else {
bool stop_tick = true;

+ tick_nohz_idle_stop_tick();
+ rcu_idle_enter();
/*
* Ask the cpuidle framework to choose a convenient idle state.
*/
next_state = cpuidle_select(drv, dev, &stop_tick);

- if (stop_tick)
- tick_nohz_idle_stop_tick();
- else
- tick_nohz_idle_retain_tick();
-
- rcu_idle_enter();
-
entered_state = call_cpuidle(drv, dev, next_state);
/*
* Give the governor an opportunity to reflect on the outcome