`isolcpus` kernel parameter has no effect on Ubuntu 16.04 desktop

ubuntu grub

13,724

I would guess those two processes are kworker or another kernel task.

It appears that presently, isolcpus= only affects user space threads, and hence, kernel threads will show up on the isolated CPU's.

From the RHEL documentation (emphasis mine),

3.3.6.2. Isolating CPUs

You can isolate one or more CPUs from the scheduler with the isolcpus boot parameter. This prevents the scheduler from scheduling any user-space threads on this CPU. Once a CPU is isolated, you must manually assign processes to the isolated CPU, either with the CPU affinity system calls or the numactl command.

Someone tried to patch this behaviour, but as far as I can tell, it wasn't acked.

In summary, Ubuntu isn't ignoring the option, it's working as intended. Whether it's working as desired is a different query.

13,724

nom

Updated on September 18, 2022

Comments

nom over 1 year
I added the following line to /etc/default/grub
```
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash isolcpus=1,3"
```
then ran sudo grub-update (and rebooted) but according to top processes are still being scheduled on CPU 1 and 3. What should I do about this?

Things I was asked to add:
```
nabeel@nabeel-Inspiron-3542:~$ cat /proc/cmdline
BOOT_IMAGE=/boot/vmlinuz-4.4.0-22-generic.efi.signed root=UUID=1e46ca65-843f-439a-8e2a-f5e666a03ffe ro quiet splash isolcpus=1,3 vt.handoff=7
```
Output of top:
```
PID USER      PR S P 
182 root      20 S 0 
183 root       0 S 0 
220 root      20 S 0 
223 root      20 S 0 
253 root      20 S 2 
261 root       0 S 0 
312 root     -51 S 0 
343 root       0 S 3 <-------- Processes being scheduled on CPU 1 and 3
347 root       0 S 1 <--------
373 root       0 S 2 
398 root       0 S 0 
474 root       0 S 2 
478 root      20 S 0 
548 systemd+  20 S 0 
632 avahi     20 S 2 
633 root      20 S 2 
642 root      20 S 0 
```
- Admin almost 8 years
  
  Did you reboot?
- Admin almost 8 years
  
  @EightBitTony Yeah, I did.
- Admin almost 8 years
  
  Can you add the output of cat /proc/cmdline to your question? And, show the output from top which shows which CPU's have processes on them.
- Admin almost 8 years
  
  What are those two processes? e.g. are they kworker or something similar?
- Admin almost 6 years
  
  I've found that .. GRUB_CMDLINE_LINUX_DEFAULT="quiet splash isolcpus=3,7" ... only works with grub2 on Skylake "client" (i.e. consumer devices)
nom almost 8 years

You are right, they are kworker threads (and there are a lot of them), but doesn't that defeat the point of isolating a CPU?
EightBitTony almost 8 years

It depends, but that's how the option works, so it is working as expected.