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Date: Tue, 17 Jul 2018 10:52:02 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Andreas Herrmann <aherrmann@...e.com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
Peter Zijlstra <peterz@...radead.org>,
Frederic Weisbecker <frederic@...nel.org>,
Viresh Kumar <viresh.kumar@...aro.org>,
Linux PM <linux-pm@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Commit 554c8aa8ecad causing severe performance degression with pcc-cpufreq
On Tue, Jul 17, 2018 at 10:36 AM, Andreas Herrmann <aherrmann@...e.com> 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@...e.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?
>
> 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.
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