[Fastboot] Documentation feedback request.
Michael Ellerman
michael at ellerman.id.au
Thu Apr 27 09:44:31 EST 2006
Hi David,
Nice work. Few comments below ...
On Wed, 2006-04-26 at 10:49 -0700, David Wilder wrote:
> Attached is a updated Documentation/kdump/kdump.txt. Please provide
> comments. I will incorporate your feedback and send up to Linus.
>
> plain text document attachment (kdump-final-20060425.txt)
> ================================================================
> Documentation for Kdump - The kexec-based Crash Dumping Solution
> ================================================================
>
> This document includes overview, setup and installation, and analysis
> information.
>
> Overview
> ========
>
> Kdump uses kexec to quickly boot to a dump-capture kernel whenever a
> dump of the system kernel's memory needs to be taken (for example, when
> the system panics). The system kernel's memory image is preserved across
> the reboot and is accessible to the dump-capture kernel.
>
> You can use common Linux commands, such as cp and scp, to copy the
> memory image to a dump file on the local disk, or across the network to
> a remote system.
>
> Kdump and kexec are currently supported on the x86, x86_64, and ppc64
> architectures.
s/ppc64/64-bit powerpc/ ?
> When the system kernel boots, it reserves a small section of memory for
> the dump-capture kernel. This ensures that ongoing Direct Memory Access
> (DMA) from the system kernel does not corrupt the dump-capture kernel.
> The kexec -p command loads the dump-capture kernel into this reserved
> memory.
Well hopefully ;)
> On x86 machines, the first 640 KB of physical memory is needed to boot,
> regardless of where the kernel loads. Therefore, kexec backs up this
> region just before rebooting into the dump-capture kernel.
>
> All of the necessary information about the system kernel's core image is
> encoded in the ELF format, and stored in a reserved area of memory
> before a crash. The physical address of the start of the ELF header is
> passed to the dump-capture kernel through the elfcorehdr= boot
> parameter.
>
> With the dump-capture kernel, you can access the memory image, or "old
> memory," in two ways:
>
> - Through a /dev/oldmem device interface. A capture utility can read the
> device file and write out the memory in raw format. This is a raw dump
> of memory. Analysis and capture tools must be intelligent enough to
> determine where to look for the right information.
>
> - Through /proc/vmcore. This exports the dump as an ELF-format file that
> you can write out using file copy commands such as cp or scp. Further,
> you can use analysis tools such as the GNU Debugger (GDB) and the Crash
> tool to debug the dump file. This method ensures that the dump pages are
> correctly ordered.
>
>
> Setup and Installation
> ======================
>
> Install kexec-tools and the Kdump patch
> ---------------------------------------
>
> 1) Login as the root user.
>
> 2) Download the kexec-tools user-space package from the following URL:
>
> http://www.xmission.com/~ebiederm/files/kexec/kexec-tools-1.101.tar.gz
>
> 3) Unpack the tarball with the tar command, as follows:
>
> tar xvpzf kexec-tools-1.101.tar.gz
>
> 4) Download the latest consolidated Kdump patch from the following URL:
>
> http://lse.sourceforge.net/kdump/
>
> (This location is being used until all the user-space Kdump patches
> are integrated with the kexec-tools package.)
>
> 5) Change to the kexec-tools-1.101 directory, as follows:
>
> cd kexec-tools-1.101
>
> 6) Apply the consolidated patch to the kexec-tools-1.101 source tree
> with the patch command, as follows. (Modify the path to the downloaded
> patch as necessary.)
>
> patch -p1 < /path-to-kdump-patch/kexec-tools-1.101-kdump.patch
>
> 7) Configure the package, as follows:
>
> ./configure
>
> 8) Compile the package, as follows:
>
> make
>
> 9) Install the package, as follows:
>
> make install
>
>
> Download and build the system and dump-capture kernels
> ------------------------------------------------------
>
> Download the mainline (vanilla) kernel source code (2.6.13-rc1 or newer)
> from http://www.kernel.org. Two kernels must be built: a system kernel
> and a dump-capture kernel. Use the following steps to configure these
> kernels with the necessary kexec and Kdump features:
>
> System kernel
> -------------
>
> 1) Enable "kexec system call" in "Processor type and features."
>
> CONFIG_KEXEC=y
>
> 2) Enable "sysfs file system support" in "Filesystem" -> "Pseudo
> filesystems." This is usually enabled by default.
>
> CONFIG_SYSFS=y
>
> Note that "sysfs file system support" might not appear in the "Pseudo
> filesystems" menu if "Configure standard kernel features (for small
> systems)" is not enabled in "General Setup." In this case, check the
> .config file itself to ensure that sysfs is turned on, as follows:
>
> grep 'CONFIG_SYSFS' .config
Is there a particular requirement for sysfs?
> 3) Enable "Compile the kernel with debug info" in "Kernel hacking."
>
> CONFIG_DEBUG_INFO=Y
>
> This causes the kernel to be built with debug symbols. The dump
> analysis tools require a vmlinux with debug symbols in order to read
> and analyze a dump file.
>
> 4) Make and install the kernel and its modules. Update the boot loader
> (such as grub, yaboot, or lilo) configuration files as necessary.
>
> 5) Boot the system kernel with the boot parameter "crashkernel=Y at X",
> where Y specifies how much memory to reserve for the dump-capture kernel
> and X specifies the beginning of this reserved memory. For example,
> "crashkernel=64M at 16M" tells the system kernel to reserve 64 MB of memory
> starting at physical address 0x01000000 for the dump-capture kernel.
Most of this doesn't apply to powerpc.
> On x86 and x86_64, use "crashkernel=64M at 16M".
>
> On ppc64, use "crashkernel=128M at 32M".
No just use "crashkernel=128M".
> The dump-capture kernel
> -----------------------
>
> 1) Under "General setup," append "-kdump" to the current string in
> "Local version."
>
> 2) On x86, enable high memory support under "Processor type and
> features":
>
> CONFIG_HIGHMEM=y
>
> 3) On x86 and x86_64, disable symmetric multi-processing support
> under "Processor type and features":
>
> CONFIG_SMP=n
>
> 4) On ppc64, disable NUMA support and enable EMBEDDED support:
>
> CONFIG_NUMA=n
> CONFIG_EMBEDDED=y
> CONFIG_EEH=N for the dump-capture kernel
Why are we disabling NUMA? AFAIK we work on more systems with NUMA than
without?
And why are we turning off EMBEDDED and EEH?
> 5) Enable "kernel crash dumps" support under "Processor type and
> features":
>
> CONFIG_CRASH_DUMP=y
>
> 6) Use a suitable value for "Physical address where the kernel is
> loaded" (under "Processor type and features"). This only appears when
> "kernel crash dumps" is enabled. By default this value is 0x1000000
> (16MB). It should be the same as X in the "crashkernel=Y at X" boot
> parameter discussed above.
>
> On x86 and x86_64, use "CONFIG_PHYSICAL_START=0x1000000".
>
> On ppc64 the value is automatically set at 32MB when
> CONFIG_CRASH_DUMP is set.
This whole step should start "On x86 ..."
> 6) Optionally enable "/proc/vmcore support" under "Filesystems" ->
> "Pseudo filesystems".
6 + 1 = 6 :D
> CONFIG_PROC_VMCORE=y
>
> 7) Make and install the kernel and its modules. DO NOT add this kernel
> to the boot loader configuration files.
>
>
> Load the Dump-capture Kernel
> ============================
>
> After booting to the system kernel, load the dump-capture kernel using
> the following command:
>
> kexec -p <dump-capture-kernel> \
> --initrd=<initrd-for-dump-capture-kernel> --args-linux \
> --append="root=<root-dev> init 1 irqpoll"
I've never tested irqpoll on powerpc, I'm not sure we want to recommend
it, has someone tested it?
> Notes on loading the dump-capture kernel:
>
> * <dump-capture-kernel> must be a vmlinux image (that is, an
> uncompressed ELF image). bzImage does not work at this time.
>
> * By default, the ELF headers are stored in ELF64 format to support
> systems with more than 4GB memory. The --elf32-core-headers option can
> be used to force the generation of ELF32 headers. This is necessary
> because GDB currently cannot open vmcore files with ELF64 headers on
> 32-bit systems. ELF32 headers can be used on non-PAE systems (that is,
> less than 4GB of memory).
>
> * The "irqpoll" boot parameter reduces driver initialization failures
> due to shared interrupts in the dump-capture kernel.
>
> * You must specify <root-dev> in the format corresponding to the root
> device name in the output of mount command.
> * "init 1" boots the dump-capture kernel into single-user mode without
> networking. If you want networking, use "init 3."
>
>
> Kernel Panic
> ============
>
> After successfully loading the dump-capture kernel as previously
> described, the system will reboot into the dump-capture kernel if a
> panic occurs. You can write a module to force the panic, or use
> "ALT-SysRq-c" to initiate a crash dump for testing purposes.
>
>
> Write Out the Dump File
> =======================
>
> After the dump-capture kernel is booted, write out the dump file with
> the following command:
>
> cp /proc/vmcore <dump-file>
>
> You can also access dumped memory as a /dev/oldmem device for a linear
> and raw view. To create the device, use the following command:
>
> mknod /dev/oldmem c 1 12
>
> Use the dd command with suitable options for count, bs, and skip to
> access specific portions of the dump.
>
> To see the entire memory, use the following command:
>
> dd if=/dev/oldmem of=oldmem.001
>
>
> Analysis
> ========
>
> Before analyzing the dump image, you should reboot into a stable kernel.
>
> You can do limited analysis using GDB on the dump file copied out of
> /proc/vmcore. Use the debug vmlinux built with -g and run the following
> command:
>
> gdb vmlinux <dump-file>
>
> Stack trace for the task on processor 0, register display, and memory
> display work fine.
>
> Note: GDB cannot analyze core files generated in ELF64 format for x86.
> On systems with a maximum of 4GB of memory, you can generate
> ELF32-format headers using the --elf32-core-headers kernel option on the
> dump kernel.
>
> You can also use the Crash utility to analyze dump files in Kdump
> format. Crash is available on Dave Anderson's site at the following URL:
>
> http://people.redhat.com/~anderson/
>
>
> To Do
> =====
>
> 1) Provide a kernel pages filtering mechanism, so core file size is not
> extreme on systems with huge memory banks.
>
> 2) Relocatable kernel can help in maintaining multiple kernels for
> crash_dump, and the same kernel as the system kernel can be used to
> capture the dump.
>
>
> Contact
> =======
>
> Vivek Goyal (vgoyal at in.ibm.com)
> Maneesh Soni (maneesh at in.ibm.com)
fastboot at lists.osdl.org
cheers
--
Michael Ellerman
IBM OzLabs
wwweb: http://michael.ellerman.id.au
phone: +61 2 6212 1183 (tie line 70 21183)
We do not inherit the earth from our ancestors,
we borrow it from our children. - S.M.A.R.T Person
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