Magic SysRq Tutorial - What, Kernel Configuation, Use, Commands

-
What is SysRq?

SysRq is the magical key comibnation when you hit, kernel will respond regardless of whatever else it is doing, unless it is completely locked up.

Kernel Configuration to Enable SysRq?

 During Linux Kernel Compilation: CONFIG_MAGIC_SYSRQ = 'y'
While running Linux:

  • Kernel Command Line Parameter: "sysrq_always_enabled"
  • echo "number" > /proc/sys/kernel/sysrq
    • 0 - Disable sysrq completely
    • 1 - Enable all functions of sysrq
    • > 1 - Bitmask of allowed sysrq functions

How to use SysRq?

If X86: Press Key Combo: Alt + SysRq + <Command Key>. If you don't find 'SysRq' on your keyboard , use 'PrtScr', then the key combo becomes: Alt + PrtScr + <Command Key>

On All other machines ( This will be useful when there is no keyboard attached and you have a serial console) : echo <command key> > /proc/sysrq-trigger

E.g. echo b > /proc/sysrq-trigger to reboot the system
echo g > /proc/sysrq-trigger to get in debugger mode used by kgdb
echo p > /proc/sysrq-trigger to dump the current registers and flags to your console

Will it work under all conditions?

It won't work in the following conditions:
  • Kernel Panic
  • Hardware Failure preventing the kernel to run properly
Use cases
1. When the X Server is Frozen

Press the "Alt + SysRq + r" combo to take the keyboard from the X Server and then perform "CTRL + ALT+ F1" to get to the terminal

2. When the system is Frozen: "Alt + SysRq + REISUB" keys

List of commands supported:

Command
Function
b
Will immediately reboot the system without syncing or unmounting your disks.
c
Will perform a system crash by a NULL pointer dereference. A crashdump will be taken if configured.
d
Shows all locks that are held.
e
Send a SIGTERM to all processes, except for init.
f
Will call the oom killer to kill a memory hog process, but do not panic if nothing can be killed.
g
Used by kgdb (kernel debugger)
h
Will display help (actually any other key than those listed here will display help. but h is easy to remember :-)
i
Send a SIGKILL to all processes, except for init.
j
Forcibly “Just thaw it” - filesystems frozen by the FIFREEZE ioctl.
k
Secure Access Key (SAK) Kills all programs on the current virtual console. NOTE: See important comments below in SAK section.
l
Shows a stack backtrace for all active CPUs.
m
Will dump current memory info to your console.
n
Used to make RT tasks nice-able
o
Will shut your system off (if configured and supported).
p
Will dump the current registers and flags to your console.
q
Will dump per CPU lists of all armed hrtimers (but NOT regular timer_list timers) and detailed information about all clockevent devices.
r
Turns off keyboard raw mode and sets it to XLATE.
s
Will attempt to sync all mounted filesystems.
t
Will dump a list of current tasks and their information to your console.
u
Will attempt to remount all mounted filesystems read-only.
v
Forcefully restores framebuffer console
v
Causes ETM buffer dump [ARM-specific]
w
Dumps tasks that are in uninterruptable (blocked) state.
x
Used by xmon interface on ppc/powerpc platforms. Show global PMU Registers on sparc64. Dump all TLB entries on MIPS.
y
Show global CPU Registers [SPARC-64 specific]
z
Dump the ftrace buffer
0-9
Sets the console log level, controlling which kernel messages will be printed to your console. (0, for example would make it so that only emergency messages like PANICs or OOPSes would make it to your console.)


Comments

Post a Comment

Popular posts from this blog

bb.utils.contains yocto

-
Image
bb.utils.contains  is most commonly used function in Yocto. grep in poky directory returned with 696 count. $ grep -nr 'bb.utils.contains' poky/ | wc -l 696 It's definition is present in 'poky/bitbake/lib/bb/utils.py' file This function returns third argument if the second argument is subset of the first argument else returns fourth argument Let's take an example to understand this. You can see from the above screenshot, when the second argument is a subset of first argument, "present" was returned, else "notpresent" was returned We can also use bb.utils.contains inside if loop. This we can use inside a recipe. if ${@bb.utils.contains('SOMEFLAG','NEWVALUE3','true','false',d)}; then     FOO = 'HELLO' fi
Read more

Difference between RDEPENDS and DEPENDS in Yocto

-
We normally observe these two (RDEPENDS and DEPENDS) whenever we open any recipe. What should we choose when we write our own recipe. DEPENDS -> Build Time Dependency RDEPENDS -> Run Time Dependency DEPENDS: When a recipe 'A' is DEPENDS on recipe 'B'. In this case, Bitbake first builds recipe 'B' and then recipe 'A' E.g. You need 'dbus' to be built before 'wpa_supplicant'. RDEPENDS: When a recipe 'A' is RDEPENDS on recipe 'B'. In this case, Bitbake deploys 'B' on the target system when it deploys 'A'.  E.g. 'perf' RDEPENDS on 'bash' In other words, DEPENDS are those set of packages that should be available while building package, whereas RDEPENDS are set of packages that should be available during execution of the program.
Read more

make config vs oldconfig vs defconfig vs menuconfig vs savedefconfig

-
Image
Building the Linux Kernel is a two-step process: Configuring the Kernel options Building the Kernel with those options. There are many methods available for configuring the kernel. make config: Text-based Configuration.  Options are prompted one after another. All options need to be answered Access to former options is not possible make menuconfig: Menu-driven user interface Allows to navigate forwards and backward directly between features Allows to load and save files with filenames different from ".config" Provides search feature It uses ncurses library for GUI. If the ncurses library is not installed, make menuconfig option fails. To install ncurses library on Ubuntu: sudo apt-get install libncurses5-dev make defconfig: Creates a ".config" file with default options from the ARCH supplied defconfig Configurations are generally stored in the directory: arch/$(ARCH)/configs $ ls arch/x86/configs/ i386_defconfi
Read more