Firmware assisted dump (fadump) HOWTO

Introduction

Firmware assisted dump is a new feature in the 3.4 mainline kernel supported

only on powerpc architecture. The goal of firmware-assisted dump is to enable

the dump of a crashed system, and to do so from a fully-reset system, and to

minimize the total elapsed time until the system is back in production use. A

complete documentation on implementation can be found at

Documentation/powerpc/firmware-assisted-dump.txt in upstream linux kernel tree

from 3.4 version and above.

Please note that the firmware-assisted dump feature is only available on Power6

and above systems with recent firmware versions.

Overview

Fadump

Fadump is a robust kernel crash dumping mechanism to get reliable kernel crash

dump with assistance from firmware. This approach does not use kexec, instead

firmware assists in booting the kdump kernel while preserving memory contents.

Unlike kdump, the system is fully reset, and loaded with a fresh copy of the

kernel. In particular, PCI and I/O devices are reinitialized and are in a

clean, consistent state.  This second kernel, often called a capture kernel,

boots with very little memory and captures the dump image.

The first kernel registers the sections of memory with the Power firmware for

dump preservation during OS initialization. These registered sections of memory

are reserved by the first kernel during early boot. When a system crashes, the

Power firmware fully resets the system, preserves all the system memory

contents, save the low memory (boot memory of size larger of 5% of system

RAM or 256MB) of RAM to the previous registered region. It will also save

system registers, and hardware PTE's.

Fadump is supported only on ppc64 platform. The standard kernel and capture

kernel are one and the same on ppc64.

If you're reading this document, you should already have kexec-tools

installed. If not, you install it via the following command:

    # dnf install kexec-tools

Fadump Operational Flow:

Like kdump, fadump also exports the ELF formatted kernel crash dump through

/proc/vmcore. Hence existing kdump infrastructure can be used to capture fadump

vmcore. The idea is to keep the functionality transparent to end user. From

user perspective there is no change in the way kdump init script works.

However, unlike kdump, fadump does not pre-load kdump kernel and initrd into

reserved memory, instead it always uses default OS initrd during second boot

after crash. Hence, for fadump, we rebuild the new kdump initrd and replace it

with default initrd. Before replacing existing default initrd we take a backup

of original default initrd for user's reference. The dracut package has been

enhanced to rebuild the default initrd with vmcore capture steps. The initrd

image is rebuilt as per the configuration in /etc/kdump.conf file.

The control flow of fadump works as follows:

01. System panics.

02. At the crash, kernel informs power firmware that kernel has crashed.

03. Firmware takes the control and reboots the entire system preserving

    only the memory (resets all other devices).

04. The reboot follows the normal booting process (non-kexec).

05. The boot loader loads the default kernel and initrd from /boot

06. The default initrd loads and runs /init

07. dracut-kdump.sh script present in fadump aware default initrd checks if

    '/proc/device-tree/rtas/ibm,kernel-dump'  file exists  before executing

    steps to capture vmcore.

    (This check will help to bypass the vmcore capture steps during normal boot

     process.)

09. Captures dump according to /etc/kdump.conf

10. Is dump capture successful (yes goto 12, no goto 11)

11. Perform the failure action specified in /etc/kdump.conf

    (The default failure action is reboot, if unspecified)

12. Perform the final action specified in /etc/kdump.conf

    (The default final action is reboot, if unspecified)

How to configure fadump:

Again, we assume if you're reading this document, you should already have

kexec-tools installed. If not, you install it via the following command:

    # dnf install kexec-tools

Make the kernel to be configured with FADump as the default boot entry, if

it isn't already:

   # grubby --set-default=/boot/vmlinuz-<kver>

Boot into the kernel to be configured for FADump. To be able to do much of

anything interesting in the way of debug analysis, you'll also need to install

the kernel-debuginfo package, of the same arch as your running kernel, and the

crash utility:

    # dnf --enablerepo=\*debuginfo install kernel-debuginfo.$(uname -m) crash

Next up, we can enable firmware assisted dump and reserve the memory for boot

memory preservation as specified in in the table of 'FADump Memory Requirements'

section:

   # kdumpctl reset-crashkernel --fadump=on

Alternatively, you can use grubby to reserve custom amount of memory:

   # grubby --args="fadump=on crashkernel=6G" --update-kernel=/boot/vmlinuz-`uname -r`

By default, FADump reserved memory will be initialized as CMA area to make the

memory available through CMA allocator on the production kernel. We can opt out

of this, making reserved memory unavailable to production kernel, by booting the

linux kernel with 'fadump=nocma' instead of 'fadump=on':

   # kdumpctl reset-crashkernel --fadump=nocma

The term 'boot memory' means size of the low memory chunk that is required for

a kernel to boot successfully when booted with restricted memory.  By default,

the boot memory size will be the larger of 5% of system RAM or 256MB.

Alternatively, user can also specify boot memory size through boot parameter

'fadump_reserve_mem=' which will override the default calculated size. Use this

option if default boot memory size is not sufficient for second kernel to boot

successfully.

After making said changes, reboot your system, so that the specified memory is

reserved and left untouched by the normal system. Take note that the output of

'free -m' will show X MB less memory than without this parameter, which is

expected. If you see OOM (Out Of Memory) error messages while loading capture

kernel, then you should bump up the memory reservation size.

Now that you've got that reserved memory region set up, you want to turn on

the kdump init script:

    # systemctl enable kdump.service

Then, start up kdump as well:

    # systemctl start kdump.service

This should turn on the firmware assisted functionality in kernel by

echo'ing 1 to /sys/kernel/fadump_registered, leaving the system ready

to capture a vmcore upon crashing. For journaling filesystems like XFS an

additional step is required to ensure bootloader does not pick the

older initrd (without vmcore capture scripts):

  * If /boot is a separate partition, run the below commands as the root user,

    or as a user with CAP_SYS_ADMIN rights:

        # fsfreeze -f

        # fsfreeze -u

  * If /boot is not a separate partition, reboot the system.

After reboot check if the kdump service is up and running with:

  # systemctl status kdump.service

To test out whether FADump is configured properly, you can force-crash your

system by echo'ing a 'c' into /proc/sysrq-trigger:

    # echo c > /proc/sysrq-trigger

You should see some panic output, followed by the system reset and booting into

fresh copy of kernel. When default initrd loads and runs /init, vmcore should

be copied out to disk (by default, in /var/crash/<YYYY.MM.DD-HH:MM:SS>/vmcore),

then the system rebooted back into your normal kernel.

Once back to your normal kernel, you can use the previously installed crash

kernel in conjunction with the previously installed kernel-debuginfo to

perform postmortem analysis:

    # crash /usr/lib/debug/lib/modules/2.6.17-1.2621.el5/vmlinux

    /var/crash/2006-08-23-15:34/vmcore

    crash> bt

and so on...

Saving vmcore-dmesg.txt

-----------------------

Kernel log bufferes are one of the most important information available

in vmcore. Now before saving vmcore, kernel log bufferes are extracted

from /proc/vmcore and saved into a file vmcore-dmesg.txt. After

vmcore-dmesg.txt, vmcore is saved. Destination disk and directory for

vmcore-dmesg.txt is same as vmcore. Note that kernel log buffers will

not be available if dump target is raw device.

FADump Memory Requirements:

  System Memory          Recommended memory

--------------------- ----------------------

    4 GB - 16 GB     :        768 MB

   16 GB - 64 GB     :       1024 MB

   64 GB - 128 GB    :          2 GB

  128 GB - 1 TB      :          4 GB

    1 TB - 2 TB      :          6 GB

    2 TB - 4 TB      :         12 GB

    4 TB - 8 TB      :         20 GB

    8 TB - 16 TB     :         36 GB

   16 TB - 32 TB     :         64 GB

   32 TB - 64 TB     :        128 GB

   64 TB & above     :        180 GB

Things to remember:

1) The memory required to boot capture Kernel is a moving target that depends

   on many factors like hardware attached to the system, kernel and modules in

   use, packages installed and services enabled, there is no one-size-fits-all.

   But the above recommendations are based on system memory. So, the above

   recommendations for FADump come with a few assumptions, based on available

   system memory, about the resources the system could have. So, please take

   the recommendations with a pinch of salt and remember to try capturing dump

   a few times to confirm that the system is configured successfully with dump

   capturing support.

2) Though the memory requirements for FADump seem high, this memory is not

   completely set aside but made available for userspace applications to use,

   through the CMA allocator.

3) As the same initrd is used for booting production kernel as well as capture

   kernel and with dump being captured in a restricted memory environment, few

   optimizations (like not inclding network dracut module, disabling multipath

   and such) are applied while building the initrd. In case, the production

   environment needs these optimizations to be avoided, dracut_args option in

   /etc/kdump.conf file could be leveraged. For example, if a user wishes for

   network module to be included in the initrd, adding the below entry in

   /etc/kdump.conf file and restarting kdump service would take care of it.

   dracut_args --add "network"

4) If FADump is configured to capture vmcore to a remote dump target using SSH

   or NFS protocol, the corresponding network interface '<interface-name>' is

   renamed to 'kdump-<interface-name>', if it is generic (like *eth# or net#).

   It happens because vmcore capture scripts in the initial RAM disk (initrd)

   add the 'kdump-' prefix to the network interface name to secure persistent

   naming. And as capture kernel and production kernel use the same initrd in

   case of FADump, the interface name is changed for the production kernel too.

   This is likely to impact network configuration setup for production kernel.

   So, it is recommended to use a non-generic name for a network interface,

   before setting up FADump to capture vmcore to a remote dump target based on

   that network interface, to avoid running into network configuration issues.

Dump Triggering methods:

This section talks about the various ways, other than a Kernel Panic, in which

fadump can be triggered. The following methods assume that fadump is configured

on your system, with the scripts enabled as described in the section above.

1) AltSysRq C

FAdump can be triggered with the combination of the 'Alt','SysRq' and 'C'

keyboard keys. Please refer to the following link for more details:

https://fedoraproject.org/wiki/QA/Sysrq

In addition, on PowerPC boxes, fadump can also be triggered via Hardware

Management Console(HMC) using 'Ctrl', 'O' and 'C' keyboard keys.

2) Kernel OOPs

If we want to generate a dump everytime the Kernel OOPses, we can achieve this

by setting the 'Panic On OOPs' option as follows:

    # echo 1 > /proc/sys/kernel/panic_on_oops

3) PowerPC specific methods:

On IBM PowerPC machines, issuing a soft reset invokes the XMON debugger(if

XMON is configured). To configure XMON one needs to compile the kernel with

the CONFIG_XMON and CONFIG_XMON_DEFAULT options, or by compiling with

CONFIG_XMON and booting the kernel with xmon=on option.

Following are the ways to remotely issue a soft reset on PowerPC boxes, which

would drop you to XMON. Pressing a 'X' (capital alphabet X) followed by an

'Enter' here will trigger the dump.

3.1) HMC

Hardware Management Console(HMC) available on Power4 and Power5 machines allow

partitions to be reset remotely. This is specially useful in hang situations

where the system is not accepting any keyboard inputs.

Once you have HMC configured, the following steps will enable you to trigger

fadump via a soft reset:

On Power4

  Using GUI

    * In the right pane, right click on the partition you wish to dump.

    * Select "Operating System->Reset".

    * Select "Soft Reset".

    * Select "Yes".

  Using HMC Commandline

    # reset_partition -m <machine> -p <partition> -t soft

On Power5

  Using GUI

    * In the right pane, right click on the partition you wish to dump.

    * Select "Restart Partition".

    * Select "Dump".

    * Select "OK".

  Using HMC Commandline

    # chsysstate -m <managed system name> -n <lpar name> -o dumprestart -r lpar

3.2) Blade Management Console for Blade Center

To initiate a dump operation, go to Power/Restart option under "Blade Tasks" in

the Blade Management Console. Select the corresponding blade for which you want

to initate the dump and then click "Restart blade with NMI". This issues a

system reset and invokes xmon debugger.

Advanced Setups & Failure action:

Kdump and fadump exhibit similar behavior in terms of setup & failure action.

For fadump advanced setup related information see section "Advanced Setups" in

"kexec-kdump-howto.txt" document. Refer to "Failure action" section in "kexec-

kdump-howto.txt" document for fadump failure action related information.

Compression and filtering

Refer "Compression and filtering" section in "kexec-kdump-howto.txt" document.

Compression and filtering are same for kdump & fadump.

Notes on rootfs mount:

Dracut is designed to mount rootfs by default. If rootfs mounting fails it

will refuse to go on. So fadump leaves rootfs mounting to dracut currently.

We make the assumtion that proper root= cmdline is being passed to dracut

initramfs for the time being. If you need modify "KDUMP_COMMANDLINE=" in

/etc/sysconfig/kdump, you will need to make sure that appropriate root=

options are copied from /proc/cmdline. In general it is best to append

command line options using "KDUMP_COMMANDLINE_APPEND=" instead of replacing

the original command line completely.

How to disable FADump:

Remove "fadump=on"/"fadump=nocma" from kernel cmdline parameters OR replace

it with "fadump=off" kernel cmdline parameter:

   # grubby --update-kernel=/boot/vmlinuz-`uname -r` --remove-args="fadump=on"

or

   # grubby --update-kernel=/boot/vmlinuz-`uname -r` --remove-args="fadump=nocma"

OR

   # grubby --update-kernel=/boot/vmlinuz-`uname -r` --args="fadump=off"

Remove "crashkernel=" from kernel cmdline parameters:

   # grubby --update-kernel=/boot/vmlinuz-`uname -r` --remove-args="crashkernel"

If KDump is to be used as the dump capturing mechanism, reset the crashkernel parameter:

   # kdumpctl reset-crashkernel --fadump=off

Reboot the system for the settings to take effect.