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kexec-tools/fadump-howto.txt
Hari Bathini da6f381b08 fadump: improve fadump-howto.txt about remote dump target setup 
While fadump-howto.txt talks about what happens to network interface
name on setting up a remote dump target in FADump mode, it doesn't
explicitly specify the negative consequences of it. Make it explicit
and provide a recommendation to overcome the same.

Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Acked-by: Kairui Song <kasong@redhat.com>
2021-02-23 16:18:54 +08:00

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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:
# yum 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:
# yum 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:
# yum --enablerepo=\*debuginfo install kernel-debuginfo.$(uname -m) crash
Next up, we need to modify some boot parameters to enable firmware assisted
dump. With the help of grubby, it's very easy to append "fadump=on" to the end
of your kernel boot parameters. To reserve the appropriate amount of memory
for boot memory preservation, pass 'crashkernel=X' kernel cmdline parameter.
For the recommended value of X, see 'FADump Memory Requirements' section.
# 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'.
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"
If KDump is to be used as the dump capturing mechanism, update the crashkernel
parameter (Else, remove "crashkernel=" parameter too, using grubby):
# grubby --update-kernel=/boot/vmlinuz-$kver --args="crashkernl=auto"
Reboot the system for the settings to take effect.
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