isolcpus=1,2,3 does not isolate on Raspberry Pi 4Bx4Gb 5.15.65-rt49/2.9 32bit

Like the title says, when I add " isolcpus=1,2,3 idle=poll " to the end of the string at /boot/cmdline.text.
I do not get 1-3 when I cat /sys/devices/system/cpu/isolated.

When I run latency-histogram --nobase I get "4 cores no_isolcpus ARMv7 Processor rev 3 (v71) as a header. 
min(us)-108.6, sdev(us) 2.2, max(us)121.5

If I run the histogram and open htop I can see all 4 cores bouncing the loads around.

Can anyone verify isolcpus is working on their Rpi4x4gb with this or higher PreemptRT kernel.
This was compiled in the weee hours of the night last night from a vanilla RaspiOS 32bit with the gnipsel "Installing LinuxCNC on Debian 11" instructions after loading the kdoren/linux/releases PreemptRT kernel patches.

The Pi is up and running, talking nice to my 7i90, but looking like Mt.Everest on my histogram.

Thoughts? Instructions?

kdoren's procedure puts all files in a separate folder that has the same name as the kernel release.
Indeed in the file /boot/config.txt you will find the command os_prefix=$KERN/ which in your case will be 5.15.65.rt65-v7l+.

Therefore you will need to edit the /boot/5.15.65.rt65-v7l+/cmdline.txt file.
In case it is not present, you can take the content of the one present in /boot/cmdline.txt and adapt it to your needs

in any case all the instructions and the link to the ready-made image are also available here:

forum.linuxcnc.org/9-installing-linuxcnc...erry-pi-3b-pi#260915

you can check cmdline the running kernel was started with with 'cat /proc/cmdline'

Sorry for not responding sooner guys, I have been blocked from the forum for "too many unsuccessful login attempts" not my fault, something to do with spam or hackers. On my cellphone's hotspot to post this.

I tried adding the isolcpus to the other cmdline.txt seems to have fixed me up.

Thanks guys!

I was tinkering trying to reduce latency on a pi3b  and found what looks like a similar issue with isolcpus

• Raspberry  pi 3b 
• I'm running Linux raspberry pi  5.15.84-v8+ #1613  SMP PREEMPT  aarch64 GNU-Linux
•  
• compiled Run in place  as I couldn't get debuild to complete
•  
• echo -n ' isolcpus=1,2,3 idle=poll processor.max_cstate=1' | sudo tee -a /boot/cmdline.txt
• checked all ok   
• from terminal cat /boot/cmdline.txt      isolcpus=1,2,3 idle=poll processor.max_cstate=1
•  
• from terminal launched latency-histogram 
  • from terminal it says Note:  using POSIX non-realtime  
  • in histogram is says 4 cores no_isolcpus

Regards
Nick

 

nickom44 wrote:

• from terminal it says Note:  using POSIX non-realtime  

I would worry about that first. For some reason the system isn't finding your realtime kernel. 

You say you compiled from source. When you ran

./configure --with-realtime=uspace

Did the screen output suggest that it had found a realtime-capable kernel?  (You would need to scroll back to near the beginning)