Messages in this thread | | | From | "Rafael J. Wysocki" <> | Date | Tue, 17 Jul 2018 10:58:17 +0200 | Subject | Re: Commit 554c8aa8ecad causing severe performance degression with pcc-cpufreq |
| |
On Tue, Jul 17, 2018 at 10:50 AM, Andreas Herrmann <aherrmann@suse.com> wrote: > 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.
OK
> 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).
I see. :-)
> So should the driver be removed (sooner or later), or this behaviour > be documented somewhere, or just leave it as is.
At least it should be documented in the driver that it is not scalable and not for use on many-CPU systems (with "many" meaning anything greater than 4 probably).
Or we could just make the driver not load if the number of CPUs in the system is greater than 4 or similar.
| |