Bug 255360

Summary: CPU History chart does not work well with CPU Hotswapping
Product: [Applications] plasma-systemmonitor Reporter: bluescreenavenger
Component: generalAssignee: KSysGuard Developers <ksysguard-bugs>
Status: CONFIRMED ---    
Severity: normal CC: ahiemstra, plasma-bugs-null
Priority: NOR    
Version First Reported In: master   
Target Milestone: ---   
Platform: Ubuntu   
OS: Linux   
Latest Commit: Version Fixed/Implemented In:
Sentry Crash Report:

Description bluescreenavenger 2010-10-27 01:13:21 UTC 
Version:           4.0 (using KDE 4.5.1) 
OS:                Linux

Ksysguard cpu history chart does not respond well to linux's hotswappable CPU's.


It seems ksysguard does detect cpu's going and coming back with the # of processors, or # of cores widget, its just not implanted the the CPU Usage History widget.

This could effect laptop users who have power management software that can turn off CPU's to save power


Reproducible: Always

Steps to Reproduce:
before starting ksysgaurd run
bash -c "echo 0 >> /sys/devices/system/cpu/cpu1/online"
on a dual core system to disable core #2. 


notice that there is only 1 CPU.

with ksysguard running, run
bash -c "echo 1 >> /sys/devices/system/cpu/cpu1/online"
to reenable the CPU. Notice that there remains one CPU.

Close ksysgaurd, and reopen it. notice there are two CPUs.

run bash -c "echo 0 >> /sys/devices/system/cpu/cpu1/online" to disable the CPU. notice that the that CPU's measurement freezes at its last measurement, until you run bash -c "echo 1 >> /sys/devices/system/cpu/cpu1/online" to renable the CPU.



Expected Results:  
When a CPU is deactivated, I expect the meter for the CPU to disappear, and its line on the line graph to end, and stop measuring.

When a CPU is activated, I expect a meter for that CPU to appear, and a line start measuring the CPU's usage

OS: Linux (i686) release 2.6.35-22-generic
Compiler: cc
Comment 1 Justin Zobel 2022-10-19 22:10:59 UTC 
Thank you for reporting this bug in KDE software. As it has been a while since this issue was reported, can we please ask you to see if you can reproduce the issue with a recent software version?

If you can reproduce the issue, please change the status to "CONFIRMED" when replying. Thank you!
Comment 2 bluescreenavenger 2022-10-20 02:03:04 UTC 
It still happens, even with plasma-systemmonitor 
sending 0 to
/sys/devices/system/cpu/cpu1/online
and the line graph stays at the last percentage of usage that it was before the CPU is disabled

restarting it and sending 1 to 
/sys/devices/system/cpu/cpu1/online
and it doesn't add the CPU that comes online

I guess it is a pretty rare use case, I mean, you can hot add and remove CPUs from some VMs, but that's about it
Comment 3 bluescreenavenger 2024-03-02 03:36:48 UTC 
It seems that the new plasma-systemmonitor still does this, it doesn't detect CPUs being added/removed.

Although when I filed this in 2010 I thought by now that power management daemons disabling CPUs in laptops would be more common than they actually are...