How to verify list of "isolated" and "nohz_full" CPU information from sysfs?

Solution Verified - Updated -

Environment

  • Red Hat Enterprise Linux 7.2 and higher(kernel-3.10.0-327.el7.x86_64 and higher)
  • Red Hat Enterprise Linux 8
  • Red Hat Enterprise Linux 9
  • Red Hat Enterprise Linux 10

Issue

  • How to verify list of "isolated" and "nohz_full" CPU information from sysfs?

Resolution

  • From RHEL 7.2 onwards (kernel-3.10.0-327.el7.x86_64 and higher), the isolated CPUs and nohz_full CPU information can be verified from sysfs. The information is exported under following files.
/sys/devices/system/cpu/isolated
/sys/devices/system/cpu/nohz_full
  • If isolcpus is used with managed_irq or nohz flag, then make sure to use domain flag to Isolate from the general SMP balancing and scheduling algorithms. If isolcpus is used only with managed_irq or nohz flag and cpu-list, then /sys/devices/system/cpu/isolated file shows blank output.
# cat /proc/cmdline 
BOOT_IMAGE=(hd0,gpt2)/vmlinuz-4.18.0-513.9.1.el8_9.x86_64 root=/dev/mapper/rhel_dhcp182--241-root ro crashkernel=auto resume=/dev/mapper/rhel_dhcp182--241-swap rd.lvm.lv=rhel_dhcp182-241/root rd.lvm.lv=rhel_dhcp182-241/swap isolcpus=nohz,2-7,10-15,18-23

# cat /sys/devices/system/cpu/isolated 
<null>

# cat /proc/cmdline 
BOOT_IMAGE=(hd0,gpt2)/vmlinuz-4.18.0-513.9.1.el8_9.x86_64 root=/dev/mapper/rhel_dhcp182--241-root ro crashkernel=auto resume=/dev/mapper/rhel_dhcp182--241-swap rd.lvm.lv=rhel_dhcp182-241/root rd.lvm.lv=rhel_dhcp182-241/swap isolcpus=nohz,domain,2-7,10-15,18-23

# cat /sys/devices/system/cpu/isolated 
2-7,10-15,18-23

Root Cause

Diagnostic Steps

  • Verification of CPU isolation from sysfs.
# cat /proc/cmdline 
BOOT_IMAGE=/vmlinuz-3.10.0-320.el7.x86_64 root=/dev/mapper/rhel-root ro crashkernel=auto rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap console=ttyS1,115200n81 isolcpus=1,5-8 nohz_full=2,7-10
# cat /sys/devices/system/cpu/isolated 
1,5-8
# cat /sys/devices/system/cpu/nohz_full 
2,7-10

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