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360 lines
15 KiB
360 lines
15 KiB
Firmware assisted dump (fadump) HOWTO
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Introduction
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Firmware assisted dump is a new feature in the 3.4 mainline kernel supported
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only on powerpc architecture. The goal of firmware-assisted dump is to enable
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the dump of a crashed system, and to do so from a fully-reset system, and to
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minimize the total elapsed time until the system is back in production use. A
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complete documentation on implementation can be found at
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Documentation/powerpc/firmware-assisted-dump.txt in upstream linux kernel tree
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from 3.4 version and above.
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Please note that the firmware-assisted dump feature is only available on Power6
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and above systems with recent firmware versions.
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Overview
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Fadump
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Fadump is a robust kernel crash dumping mechanism to get reliable kernel crash
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dump with assistance from firmware. This approach does not use kexec, instead
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firmware assists in booting the kdump kernel while preserving memory contents.
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Unlike kdump, the system is fully reset, and loaded with a fresh copy of the
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kernel. In particular, PCI and I/O devices are reinitialized and are in a
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clean, consistent state. This second kernel, often called a capture kernel,
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boots with very little memory and captures the dump image.
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The first kernel registers the sections of memory with the Power firmware for
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dump preservation during OS initialization. These registered sections of memory
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are reserved by the first kernel during early boot. When a system crashes, the
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Power firmware fully resets the system, preserves all the system memory
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contents, save the low memory (boot memory of size larger of 5% of system
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RAM or 256MB) of RAM to the previous registered region. It will also save
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system registers, and hardware PTE's.
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Fadump is supported only on ppc64 platform. The standard kernel and capture
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kernel are one and the same on ppc64.
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If you're reading this document, you should already have kexec-tools
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installed. If not, you install it via the following command:
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# dnf install kexec-tools
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Fadump Operational Flow:
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Like kdump, fadump also exports the ELF formatted kernel crash dump through
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/proc/vmcore. Hence existing kdump infrastructure can be used to capture fadump
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vmcore. The idea is to keep the functionality transparent to end user. From
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user perspective there is no change in the way kdump init script works.
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However, unlike kdump, fadump does not pre-load kdump kernel and initrd into
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reserved memory, instead it always uses default OS initrd during second boot
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after crash. Hence, for fadump, we rebuild the new kdump initrd and replace it
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with default initrd. Before replacing existing default initrd we take a backup
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of original default initrd for user's reference. The dracut package has been
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enhanced to rebuild the default initrd with vmcore capture steps. The initrd
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image is rebuilt as per the configuration in /etc/kdump.conf file.
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The control flow of fadump works as follows:
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01. System panics.
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02. At the crash, kernel informs power firmware that kernel has crashed.
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03. Firmware takes the control and reboots the entire system preserving
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only the memory (resets all other devices).
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04. The reboot follows the normal booting process (non-kexec).
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05. The boot loader loads the default kernel and initrd from /boot
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06. The default initrd loads and runs /init
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07. dracut-kdump.sh script present in fadump aware default initrd checks if
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'/proc/device-tree/rtas/ibm,kernel-dump' file exists before executing
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steps to capture vmcore.
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(This check will help to bypass the vmcore capture steps during normal boot
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process.)
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09. Captures dump according to /etc/kdump.conf
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10. Is dump capture successful (yes goto 12, no goto 11)
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11. Perform the failure action specified in /etc/kdump.conf
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(The default failure action is reboot, if unspecified)
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12. Perform the final action specified in /etc/kdump.conf
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(The default final action is reboot, if unspecified)
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How to configure fadump:
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Again, we assume if you're reading this document, you should already have
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kexec-tools installed. If not, you install it via the following command:
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# dnf install kexec-tools
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Make the kernel to be configured with FADump as the default boot entry, if
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it isn't already:
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# grubby --set-default=/boot/vmlinuz-<kver>
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Boot into the kernel to be configured for FADump. To be able to do much of
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anything interesting in the way of debug analysis, you'll also need to install
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the kernel-debuginfo package, of the same arch as your running kernel, and the
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crash utility:
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# dnf --enablerepo=\*debuginfo install kernel-debuginfo.$(uname -m) crash
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Next up, we can enable firmware assisted dump and reserve the memory for boot
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memory preservation as specified in in the table of 'FADump Memory Requirements'
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section:
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# kdumpctl reset-crashkernel --fadump=on
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Alternatively, you can use grubby to reserve custom amount of memory:
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# grubby --args="fadump=on crashkernel=6G" --update-kernel=/boot/vmlinuz-`uname -r`
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By default, FADump reserved memory will be initialized as CMA area to make the
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memory available through CMA allocator on the production kernel. We can opt out
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of this, making reserved memory unavailable to production kernel, by booting the
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linux kernel with 'fadump=nocma' instead of 'fadump=on':
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# kdumpctl reset-crashkernel --fadump=nocma
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The term 'boot memory' means size of the low memory chunk that is required for
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a kernel to boot successfully when booted with restricted memory. By default,
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the boot memory size will be the larger of 5% of system RAM or 256MB.
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Alternatively, user can also specify boot memory size through boot parameter
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'fadump_reserve_mem=' which will override the default calculated size. Use this
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option if default boot memory size is not sufficient for second kernel to boot
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successfully.
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After making said changes, reboot your system, so that the specified memory is
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reserved and left untouched by the normal system. Take note that the output of
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'free -m' will show X MB less memory than without this parameter, which is
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expected. If you see OOM (Out Of Memory) error messages while loading capture
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kernel, then you should bump up the memory reservation size.
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Now that you've got that reserved memory region set up, you want to turn on
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the kdump init script:
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# systemctl enable kdump.service
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Then, start up kdump as well:
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# systemctl start kdump.service
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This should turn on the firmware assisted functionality in kernel by
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echo'ing 1 to /sys/kernel/fadump/registered, leaving the system ready
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to capture a vmcore upon crashing. For journaling filesystems like XFS an
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additional step is required to ensure bootloader does not pick the
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older initrd (without vmcore capture scripts):
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* If /boot is a separate partition, run the below commands as the root user,
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or as a user with CAP_SYS_ADMIN rights:
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# fsfreeze -f
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# fsfreeze -u
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* If /boot is not a separate partition, reboot the system.
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After reboot check if the kdump service is up and running with:
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# systemctl status kdump.service
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To test out whether FADump is configured properly, you can force-crash your
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system by echo'ing a 'c' into /proc/sysrq-trigger:
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# echo c > /proc/sysrq-trigger
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You should see some panic output, followed by the system reset and booting into
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fresh copy of kernel. When default initrd loads and runs /init, vmcore should
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be copied out to disk (by default, in /var/crash/<YYYY.MM.DD-HH:MM:SS>/vmcore),
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then the system rebooted back into your normal kernel.
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Once back to your normal kernel, you can use the previously installed crash
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kernel in conjunction with the previously installed kernel-debuginfo to
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perform postmortem analysis:
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# crash /usr/lib/debug/lib/modules/2.6.17-1.2621.el5/vmlinux
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/var/crash/2006-08-23-15:34/vmcore
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crash> bt
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and so on...
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Saving vmcore-dmesg.txt
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-----------------------
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Kernel log bufferes are one of the most important information available
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in vmcore. Now before saving vmcore, kernel log bufferes are extracted
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from /proc/vmcore and saved into a file vmcore-dmesg.txt. After
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vmcore-dmesg.txt, vmcore is saved. Destination disk and directory for
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vmcore-dmesg.txt is same as vmcore. Note that kernel log buffers will
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not be available if dump target is raw device.
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FADump Memory Requirements:
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System Memory Recommended memory
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--------------------- ----------------------
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4 GB - 16 GB : 768 MB
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16 GB - 64 GB : 1024 MB
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64 GB - 128 GB : 2 GB
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128 GB - 1 TB : 4 GB
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1 TB - 2 TB : 6 GB
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2 TB - 4 TB : 12 GB
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4 TB - 8 TB : 20 GB
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8 TB - 16 TB : 36 GB
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16 TB - 32 TB : 64 GB
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32 TB - 64 TB : 128 GB
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64 TB & above : 180 GB
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Things to remember:
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1) The memory required to boot capture Kernel is a moving target that depends
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on many factors like hardware attached to the system, kernel and modules in
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use, packages installed and services enabled, there is no one-size-fits-all.
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But the above recommendations are based on system memory. So, the above
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recommendations for FADump come with a few assumptions, based on available
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system memory, about the resources the system could have. So, please take
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the recommendations with a pinch of salt and remember to try capturing dump
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a few times to confirm that the system is configured successfully with dump
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capturing support.
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2) Though the memory requirements for FADump seem high, this memory is not
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completely set aside but made available for userspace applications to use,
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through the CMA allocator.
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3) As the same initrd is used for booting production kernel as well as capture
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kernel and with dump being captured in a restricted memory environment, few
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optimizations (like not inclding network dracut module, disabling multipath
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and such) are applied while building the initrd. In case, the production
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environment needs these optimizations to be avoided, dracut_args option in
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/etc/kdump.conf file could be leveraged. For example, if a user wishes for
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network module to be included in the initrd, adding the below entry in
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/etc/kdump.conf file and restarting kdump service would take care of it.
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dracut_args --add "network"
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4) If FADump is configured to capture vmcore to a remote dump target using SSH
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or NFS protocol, the corresponding network interface '<interface-name>' is
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renamed to 'kdump-<interface-name>', if it is generic (like *eth# or net#).
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It happens because vmcore capture scripts in the initial RAM disk (initrd)
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add the 'kdump-' prefix to the network interface name to secure persistent
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naming. And as capture kernel and production kernel use the same initrd in
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case of FADump, the interface name is changed for the production kernel too.
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This is likely to impact network configuration setup for production kernel.
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So, it is recommended to use a non-generic name for a network interface,
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before setting up FADump to capture vmcore to a remote dump target based on
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that network interface, to avoid running into network configuration issues.
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Dump Triggering methods:
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This section talks about the various ways, other than a Kernel Panic, in which
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fadump can be triggered. The following methods assume that fadump is configured
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on your system, with the scripts enabled as described in the section above.
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1) AltSysRq C
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FAdump can be triggered with the combination of the 'Alt','SysRq' and 'C'
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keyboard keys. Please refer to the following link for more details:
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https://fedoraproject.org/wiki/QA/Sysrq
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In addition, on PowerPC boxes, fadump can also be triggered via Hardware
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Management Console(HMC) using 'Ctrl', 'O' and 'C' keyboard keys.
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2) Kernel OOPs
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If we want to generate a dump everytime the Kernel OOPses, we can achieve this
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by setting the 'Panic On OOPs' option as follows:
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# echo 1 > /proc/sys/kernel/panic_on_oops
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3) PowerPC specific methods:
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On IBM PowerPC machines, issuing a soft reset invokes the XMON debugger(if
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XMON is configured). To configure XMON one needs to compile the kernel with
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the CONFIG_XMON and CONFIG_XMON_DEFAULT options, or by compiling with
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CONFIG_XMON and booting the kernel with xmon=on option.
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Following are the ways to remotely issue a soft reset on PowerPC boxes, which
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would drop you to XMON. Pressing a 'X' (capital alphabet X) followed by an
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'Enter' here will trigger the dump.
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3.1) HMC
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Hardware Management Console(HMC) available on Power4 and Power5 machines allow
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partitions to be reset remotely. This is specially useful in hang situations
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where the system is not accepting any keyboard inputs.
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Once you have HMC configured, the following steps will enable you to trigger
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fadump via a soft reset:
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On Power4
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Using GUI
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* In the right pane, right click on the partition you wish to dump.
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* Select "Operating System->Reset".
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* Select "Soft Reset".
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* Select "Yes".
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Using HMC Commandline
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# reset_partition -m <machine> -p <partition> -t soft
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On Power5
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Using GUI
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* In the right pane, right click on the partition you wish to dump.
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* Select "Restart Partition".
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* Select "Dump".
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* Select "OK".
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Using HMC Commandline
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# chsysstate -m <managed system name> -n <lpar name> -o dumprestart -r lpar
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3.2) Blade Management Console for Blade Center
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To initiate a dump operation, go to Power/Restart option under "Blade Tasks" in
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the Blade Management Console. Select the corresponding blade for which you want
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to initate the dump and then click "Restart blade with NMI". This issues a
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system reset and invokes xmon debugger.
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Advanced Setups & Failure action:
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Kdump and fadump exhibit similar behavior in terms of setup & failure action.
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For fadump advanced setup related information see section "Advanced Setups" in
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"kexec-kdump-howto.txt" document. Refer to "Failure action" section in "kexec-
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kdump-howto.txt" document for fadump failure action related information.
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Compression and filtering
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Refer "Compression and filtering" section in "kexec-kdump-howto.txt" document.
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Compression and filtering are same for kdump & fadump.
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Notes on rootfs mount:
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Dracut is designed to mount rootfs by default. If rootfs mounting fails it
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will refuse to go on. So fadump leaves rootfs mounting to dracut currently.
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We make the assumtion that proper root= cmdline is being passed to dracut
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initramfs for the time being. If you need modify "KDUMP_COMMANDLINE=" in
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/etc/sysconfig/kdump, you will need to make sure that appropriate root=
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options are copied from /proc/cmdline. In general it is best to append
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command line options using "KDUMP_COMMANDLINE_APPEND=" instead of replacing
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the original command line completely.
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How to disable FADump:
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Remove "fadump=on"/"fadump=nocma" from kernel cmdline parameters OR replace
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it with "fadump=off" kernel cmdline parameter:
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# grubby --update-kernel=/boot/vmlinuz-`uname -r` --remove-args="fadump=on"
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or
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# grubby --update-kernel=/boot/vmlinuz-`uname -r` --remove-args="fadump=nocma"
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OR
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# grubby --update-kernel=/boot/vmlinuz-`uname -r` --args="fadump=off"
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Remove "crashkernel=" from kernel cmdline parameters:
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# grubby --update-kernel=/boot/vmlinuz-`uname -r` --remove-args="crashkernel"
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If KDump is to be used as the dump capturing mechanism, reset the crashkernel parameter:
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# kdumpctl reset-crashkernel --fadump=off
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Reboot the system for the settings to take effect.
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