I have an old machine running Linux kernel 18.104.22.168 which is behaving oddly. I suspect ONE of the CPUs is possibly heating faster than the other. I want to run some tests with specific CPUs/cores disabled and enabled. I googled for directions, and the only matches I find are number of CPUs at boot time via kernel parameters, or setting /sys/devices/system/cpu/cpuX/online with 0 or 1. But /sys/devices/system/cpu/cpuX/online does not exist. However, /sys/devices/system/cpu/online does. I re-googled for that path, but Google insists on matching pages with /sys/devices/system/cpu/cpuX/online anyway (doesn't highlight the cpuX part). Searching kernel source finds one document that has the path, but no explanation of what to set. The current contents of /sys/devices/system/cpu/online is "0-3". I'd like to know what value to set here for each different CPU to test, possibly testing all 15 combinations of on/off. This machine is a dual-socket each with "Dual-Core AMD Opteron(tm) Processor 2220" at 2.8 GHz.
Note: What you want works with hotplug enabled systems only.
If the directory does not exist, means that you cannot hotplug the core, as you whish.
There is a grub option that just can limit the number of cores, but you cannot select specific ones to use.
The other option is, to check for bios abilities. See comment.
In the end you could use a newer (live) kernel for the load testing.