Posts on Linux

Tutorials and tips on Linux system configuration and administration, and software for Linux systems.

A Beginners’ Guide to x86-64 Instruction Encoding

By Eric Z Ma,
The encoding of x86 and x86-64 instructions is well documented in Intel or AMD’s manuals. However, they are not quite easy for beginners to start with to learn encoding of the x86-64 instructions. In this post, I will give a list of useful manuals for understanding and studying the x86-64 instruction encoding, a brief introduction and an example to help you get started with the formats and encodings of the x86-64 instructions. » Read more

How To Debug Linux Kernel With Less Efforts

By Weiwei Jia,
Introduction In general, if we want to debug Linux Kernel, there are lots of tools such as Linux Perf, Kprobe, BCC, Ktap, etc, and we can also write kernel modules, proc subsystems or system calls for some specific debugging aims. However, if we have to instrument kernel to achieve our goals, usually we would not like to pay more efforts like above solutions since we’d like to achieve our aims quickly and easily. » Read more

How to Compile and Install Linux Kernel from the Source Code in an Existing Linux OS

By Eric Z Ma,
Building Linux kernel may sound a complex and geek-only thing. However, as Linux kernel itself has much less depended tools/packages compared to other software packages, it is quite easy to compile, build and install a Linux kernel from the source code in an existing Linux OS. Building Linux kernel is needed if you need to build a specific Linux kernel or update your drivers for a kernel by yourself. » Read more

How does linux kernel collect task stats data

By Weiwei Jia,
Motivation ∞ Recently, I find it is hard to know the percentage of time that one process uses to wait for synchronous I/O (eg, read, etc). One way is to use the taskstats API provided by Linux Kernel [1]. However, for this way, the precision may be one problem. With this problem, I dig into Linux Kernel source codes to see how “blkio_delay_total” (Delay time waiting for synchronous block I/O to complete) is calculated. » Read more

Vim Tutorial for Beginners: vimtutor

By Eric Z Ma,
There are many Vim tutorials and Vim tips on the Web. However, I find the vimtutor provides the best tutorial among those so far as I found on the Web while the vimtutor seems usually reachable from a terminal which is not obviously known to Vim beginners who are usually Linux beginners too. This page shows the content of the Vim tutorial from the command vimtutor in a web page which is especially easier for Vim beginners to get and learn. » Read more

x-data-plane feature in QEMU/KVM

By Weiwei Jia,
Abstract In systems, sometimes, we use one global lock to keep synchronization among different threads. This principle also happens in QEMU/KVM (http://wiki.qemu.org/Main_Page) system. However, this may cause lock contention problem. The performance/scalability of whole system will be decreased. In order to solve this problem in QEMU/KVM, x-data-plane feature is designed/implemented, which the high-level idea is “I/O requests are handled by dedicated IOThread rather than QEMU main loop threads so that it will not have lock contention among I/O threads and other QEMU main loop threads”. » Read more

The length of timeslices for processes under CFS process scheduling algorithm in Linux Kernel

By Weiwei Jia,
Abstract As is known, CFS (Completely Fair Scheduling) is a famous process scheduling algorithm in Linux Kernel but there is no convenient way for developers to get the timeslices of processes if CFS is chosen. In this article, I will introduce one way to hack the timeslices of process easily for CFS in Linux Kernel. Note that, the way introduced following is under Linux Kernel 3.16.39. » Read more

How sched_min_granularity_ns, sched_latency_ns and sched_wakeup_granularity_ns in CFS affect the timeslice of processes

By Weiwei Jia,
Abstract Currently, the most famous process scheduling algorithm in Linux Kernel is Completely Fair Scheduling (CFS) algorithm. The core idea of CFS is to let each process share the same proportional CPU resources to run so that it is fair to each process. In this article, I will introduce how sched_min_granularity_ns and sched_latency_ns work internal CFS to affect the timeslice of processes. » Read more

How migration thread works inside of Linux Kernel

By Weiwei Jia,
Abstract In computer systems, resources have to be balanced so that the performance will be better based on the same hardware. In Linux Kernel system, we will see some migration kernel threads running as daemons to do this kind of jobs as follows. In this article, we will discuss how Linux Kernel balances its hardware/software resources (NOTE: this article is based on Linux Kernel 4.7 and other versions may have some differences). » Read more

How sched_setaffinity works inside of Linux Kernel

By Weiwei Jia,
Abstract Sometimes, we may want to migrate one process/thread to one specific CPU for some specific purpose. In the Unix/Linux systems, you may choose sched_setaffinity to finish this job. This article will help you to understand how sched_setaffinity (or other APIs like pthread_setaffinity_np in user-space) works internal Linux kernel. Details -- sched_setaffinity(pid_t pid, const struct cpumask *in_mask) --- __set_cpus_allowed_ptr(struct task_struct *p, const struct cpumask *new_mask, bool check) ---- stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg) ----- migration_cpu_stop(void *data) ------ __migrate_task(struct rq *rq, struct task_struct *p, int dest_cpu) ------- move_queued_task(struct rq *rq, struct task_struct *p, int new_cpu) -------- enqueue_task(struct rq *rq, struct task_struct *p, int flags) --------- returns the new run queue of destination CPU Above character steps give a workflow of how sched_setaffinity works (how it migrates one process/thread from the run queue of source CPU to the run queue of destination CPU). » Read more

How to Install Wine 32-bit on CentOS 7

By Eric Z Ma,
Since version 7, RHEL has only x86-64 versions. The same thing happens to CentOS 7. In CentOS 7/EPEL, there is only package for Wine x86-64. However, many Windows .exe files are 32-bit. Even there are 64-bit versions for some software, their installation file is 32-bit. And for some certain software such as Office 2007, 32-bit wine is preferred. In this post, we will check how to install 32-bit Wine on CentOS 7. » Read more

How to Change Systemd Boot Target on Linux

By Eric Z Ma,
Many Linux distros, such as RHEL/CentOS 7, Fedora, Ubuntu 16, are now using systemd instead of init as the init system. It is common for Linux users to set Linux to boot to “GUI” or “Text” mode. The old way of changing ‘/etc/inittab’ for choosing Linux runlevels is not working for sytemd. This post will introduce the way for systemd systems to select the “runlevels”. » Read more

How to Change Linux Account Password Through SSH: A Beginners’ Tutorial

By Eric Z Ma,
People are sometimes given access to Linux/Unix and asked to change their initial passwords. But for beginners, changing a Linux/Unix password is not an easy task, especially when there is only SSH log on allowed to the Linux/Unix server. This post introduces how to change password of user USER on host HOST remotely using SSH. linux-passwd.png Steps are as follows. Note: you will need to replace HOST and USER with the actual username and hostname you are using. » Read more

How to Force Linux to Unmount a Filesystem Reporting “device is busy”

By Eric Z Ma,
Linux may report “device is busy” when we try to umount a filesystem. This behavior is reasonable as it can help us avoid data loss by disallowing unmouting a filesystem when it is being used. But for situations when we are sure there is something wrong happened or we care not data lost such as a NFS mounting failed because that the NFS server is dead and will never be back. » Read more

How to Disable and Enable Laptop Keyboard in Linux

By Eric Z Ma,
Attaching a USB keyboard to a laptop is common when using a laptop because a normal keyboard may provide a more convenient typing experience. The laptop keyboard is not used in these situations. However, the laptop keyboard may still be touched by accident. In this post, we will discuss how to disable and enable the laptop keyboard in Linux. I will also provide scripts ready for you to directly use them for disabling/enabling the laptop keyboard. » Read more

How to Set the Static IP Address Using CLI in Fedora/CentOS Linux

By Eric Z Ma,
How to set the static IP address for CentOS 7/Fedora 22+ is introduced in this post. Here, we assume the Linux is using NetworkManager to manage the network. All steps are done as root. Find the interface you want to set the address ∞ Command ifconfig -a lists all interfaces. Find the interface name to set, such as enp2s0. datacenter-servers.jpg Edit the interface’s configuration file ∞ For interface NAME, the configuration file is stored in /etc/sysconfig/network-scripts/ifcfg-NAME. » Read more

How to Add Google to Firefox in Linux Mint as Default Search Engine

By Eric Z Ma,
In Firefox on Linux Mint, Google is not in the default list of search engines. Linux Mint has its criteria for adding search engines while Google seems not in those suggested by Linux Mint because “Amongst commercial search engines, only the ones which share with Linux Mint the revenue Linux Mint users generate for them are included by default” while Google seems not share revenue with Linux Mint. » Read more

How to Configure the Latest Version of Flash on Firefox for Linux: by the Fresh Player Plugin

By Eric Z Ma,
As you may know, Adobe stopped supporting the NPAPI version of Flash on Linux and the latest NPAPI version of Flash player at version 11.2 released back in 2012 will only receive security fixes. Even the security fixes to the 11.2 version of the Flash Plugin your Firefox is using will end on May 4, 2017, five years from the release date. » Read more

How to Keep Linux SSH Session Alive from Disconnecting – Server and Client Side Fixes

By Eric Z Ma,
SSH is a very common tool for Linux/Unix platforms. One annoying problem when using SSH is that the connection may get disconnected if the SSH connection is idle for some time under common configurations. Users may run an infinite loop like while true; do uptime; sleep 30; done when there is no work to be done in a SSH session. There are better ways as SSH servers and clients already provide such support and we will discuss these methods in this post. » Read more