Linux Kernel

The Linux kernel is the main component of Photon OS and is the core interface between a computer’s hardware and its processes. It communicates between the two, managing resources as efficiently as possible.

Kernel Flavours and Versions

The following list contains the different Linux kernel flavours available:

• linux - A generic kernel designed to run everywhere and support everything.
• linux-esx - Optimized to run only on VMware hypervisor (ESXi, WS, Fusion). It has minimal set of device drivers to support VMware virtual devices. uname -r displays Linux . For additional features switch to the generic flavour.
• linux-secure - Security hardened variant of the generic kernel. uname -r displays -secure suffix.
• linux-rt - This is a Photon Real Time kernel. uname -r displays -rt suffix.
• linux-aws - Optimized for AWS hypervisor kernel. uname -r displays -aws suffix.

To see the version of kernel installed, run the following command:

# rpm -qa | grep -e "^linux\(\|-esx\|-secure\|rt\|aws\)-[[:digit:]]"
linux-4.9.111-1.ph2.x86_64
linux-esx-4.9.111-1.ph2.x86_64

To see the version of the Kernel that is running currently, run the following command:

# uname -r
4.9.107-1.ph2-esx

From the output, you can see that the kernel running currently doesn’t match the installer. This happens when linux-* rpms were updated but was not restarted. Restart is required.

Configuration

To find the configurations of the installed Kernel, check the /boot directory by running the following command:

# ls /boot/config-*
config-4.9.111-1.ph2 config-4.9.111-1.ph2-esx

To get a copy of the kernel configuration (Not all flavours support this feature), run the zcat /proc/config.gz command.

Boot Parameters and initrd

Several kernel flavors can be installed on the system, but only one is used during boot. /boot/photon.cfg symlink points to the kernel which is used for boot.

# ls -l /boot/photon.cfg
lrwxrwxrwx 1 root root 23 Jun 12  2018 /boot/photon.cfg -> linux-4.9.111-1.ph2.cfg

Its contents can be checked by running the following command:

# cat /boot/photon.cfg

# GRUB Environment Block

photon_cmdline=init=/lib/systemd/systemd ro loglevel=3 quiet no-vmw-sta

photon_linux=vmlinuz-4.9.111-1.ph2

photon_initrd=initrd.img-4.9.111-1.ph2

Where:

• photon_cmdline - Kernel parameters. This list will be extended by values from /boot/systemd.cfg file and the values are hardcoded to /boot/grub2/grub.cfg file (For example: root=).
• photon_linux - Kernel image to boot.
• photon_initrd - Initrd to use at boot.

Parameters of the kernel loading currently can be found by running the /proc/cmdline command:

# cat /proc/cmdline

BOOT_IMAGE=/boot/vmlinuz-4.9.107-1.ph2-esx root=PARTUUID=29194d05-4a6e-4e0c-b1f4-5020e5e8472c net.ifnames=0 init=/lib/systemd/systemd ro loglevel=3 quiet no-vmw-sta

Dmesg

To view message buffer of the kernel run the dmesg command.

Sysctl State

To view a list of all active units run the systemctl list-units command.

Kernel Statistics

The kernel statistics can be found by running the following commands:

• procfs
• sysfs
• debugfs

Kernel Modules

To view the kernel log buffer run the journalctl -k command.

To view a list of available kernel modules run the lsmod command.

To view detailed information about all connected PCI buses run the lspci command.