Wednesday, October 18, 2017

Oracle Linux - Check your kernel modules

Knowing and understanding what is running on your Oracle Linux system is vital for proper maintenance and proper tuning. As operating systems are seen more and more as something that is just there and should not be hindrance for development, as we see the rise of container based solutions and serverless computing it might look like that the operating system becomes less and less important. However, the opposite is true, the operating system becomes more and more important as it need to be able to facilitate all the requirements from the containers and functions running on top of it without human intervention or at least as less human intervention as possible.

This brings that, if you operate a large deployment of servers and you have to ensure everything is automated and operating at the best of performance at any moment in time without having to touch the systems or at least as less as possible, you need to optimize it and automate it. To be able to do so you need to be able to understand every component and be able to check if you need it or that you can drop it. Whatever you do not need, drop it, it can be a security risk or it can be a consumer of resources without having the need for it.

Oracle Linux Kernel modules
Kernel modules are an important part of the Oracle Linux operating system, understanding them and being able to check what is loaded and what is not should be something that you need to understand. Kernel modules are pieces of code that can be loaded and unloaded into the kernel upon demand. They extend the functionality of the kernel without the need to reboot the system.

udev
Today, all necessary modules loading is handled automatically by udev, so if you do not need to use any out-of-tree kernel modules, there is no need to put modules that should be loaded at boot in any configuration file. However, there are cases where you might want to load an extra module during the boot process, or blacklist another one for your computer to function properly.

Kernel modules can be explicitly loaded during boot and are configured as a static list in files under /etc/modules-load.d/. Each configuration file is named in the style of /etc/modules-load.d/.conf. Configuration files simply contain a list of kernel modules names to load, separated by newlines. Empty lines and lines whose first non-whitespace character is # or ; are ignored.

lsmod
Checking which kernel modules are loaded in the kernel can be done by using the lsmod command. lsmod will list all the modules. Basically it is a representation of everything you will find in the /proc/modules file however in a somewhat more understandable way. An example of the lsmod command on an Oracle Linux system running in a Vagrant box is shown below:

[root@localhost ~]# lsmod
Module                  Size  Used by
vboxsf                 38491  1 
ipv6                  391530  20 [permanent]
ppdev                   8323  0 
parport_pc             21178  0 
parport                37780  2 ppdev,parport_pc
sg                     31734  0 
pcspkr                  2094  0 
i2c_piix4              12269  0 
snd_intel8x0           33895  0 
snd_ac97_codec        127589  1 snd_intel8x0
ac97_bus                1498  1 snd_ac97_codec
snd_seq                61406  0 
snd_seq_device          4604  1 snd_seq
snd_pcm               113293  2 snd_intel8x0,snd_ac97_codec
snd_timer              26196  2 snd_seq,snd_pcm
snd                    79940  6 snd_intel8x0,snd_ac97_codec,snd_seq,snd_seq_device,snd_pcm,snd_timer
soundcore               7412  1 snd
e1000                 134545  0 
vboxvideo              42469  1 
ttm                    88927  1 vboxvideo
drm_kms_helper        120123  1 vboxvideo
drm                   343055  4 vboxvideo,ttm,drm_kms_helper
i2c_core               53097  3 i2c_piix4,drm_kms_helper,drm
vboxguest             306752  3 vboxsf,vboxvideo
sysimgblt               2595  1 vboxvideo
sysfillrect             4093  1 vboxvideo
syscopyarea             3619  1 vboxvideo
acpi_cpufreq           12697  0 
ext4                  604127  2 
jbd2                  108826  1 ext4
mbcache                 9265  1 ext4
sd_mod                 36186  3 
ahci                   26684  2 
libahci                27932  1 ahci
pata_acpi               3869  0 
ata_generic             3811  0 
ata_piix               27059  0 
video                  15828  0 
dm_mirror              14787  0 
dm_region_hash         11613  1 dm_mirror
dm_log                  9657  2 dm_mirror,dm_region_hash
dm_mod                106591  8 dm_mirror,dm_log
[root@localhost ~]# 

This could be the starting point of investigating and finding out what is loaded and what is really needed, what is not needed and what might be a good addition in some cases.

modinfo
as you might not be checking your kernel modules on a daily basis you might not know which module is used for what purpose. In this case modinfo is coming to your reseque. If you want to know, for example, what the module snd_seq is used for you can check the details with modinfo as shown in the example below.

[root@localhost ~]# modinfo snd_seq
filename:       /lib/modules/4.1.12-61.1.28.el6uek.x86_64/kernel/sound/core/seq/snd-seq.ko
alias:          devname:snd/seq
alias:          char-major-116-1
license:        GPL
description:    Advanced Linux Sound Architecture sequencer.
author:         Frank van de Pol , Jaroslav Kysela 
srcversion:     88DDA62432337CC735684EE
depends:        snd,snd-seq-device,snd-timer
intree:         Y
vermagic:       4.1.12-61.1.28.el6uek.x86_64 SMP mod_unload modversions 
parm:           seq_client_load:The numbers of global (system) clients to load through kmod. (array of int)
parm:           seq_default_timer_class:The default timer class. (int)
parm:           seq_default_timer_sclass:The default timer slave class. (int)
parm:           seq_default_timer_card:The default timer card number. (int)
parm:           seq_default_timer_device:The default timer device number. (int)
parm:           seq_default_timer_subdevice:The default timer subdevice number. (int)
parm:           seq_default_timer_resolution:The default timer resolution in Hz. (int)
[root@localhost ~]#

As you can see in the example above the snd_seq module is the Advanced Linux Sound Architecture sequencer developed by Frank van de Pol and Jaroslav Kysela. Taking this as an example, you can argue. do I need the snd_seq module if I run a server where I have no need for any sound.

Unloading "stuff" you do not need will ensure you have a faster boot sequence timing of your system, less resource consumption and as every component carries a risk of having an issue.... with less components you have theoretically less possible bugs.

In conclusion
optimizing your system by checking which kernel models should be loaded and which could be left out on your Oracle Linux system. However, when you just use it for common tasks you might not want to spend to much time on it. However, if you are building your own image or investing time in building a fully automated way of deploying servers fast in a CI/CD manner you might want to spend time on making sure only the components you really need are in the system and nothing else.


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