382 строки
17 KiB
ReStructuredText
382 строки
17 KiB
ReStructuredText
======================
|
|
Firmware-Assisted Dump
|
|
======================
|
|
|
|
July 2011
|
|
|
|
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.
|
|
|
|
- Firmware-Assisted Dump (FADump) infrastructure is intended to replace
|
|
the existing phyp assisted dump.
|
|
- Fadump uses the same firmware interfaces and memory reservation model
|
|
as phyp assisted dump.
|
|
- Unlike phyp dump, FADump exports the memory dump through /proc/vmcore
|
|
in the ELF format in the same way as kdump. This helps us reuse the
|
|
kdump infrastructure for dump capture and filtering.
|
|
- Unlike phyp dump, userspace tool does not need to refer any sysfs
|
|
interface while reading /proc/vmcore.
|
|
- Unlike phyp dump, FADump allows user to release all the memory reserved
|
|
for dump, with a single operation of echo 1 > /sys/kernel/fadump_release_mem.
|
|
- Once enabled through kernel boot parameter, FADump can be
|
|
started/stopped through /sys/kernel/fadump_registered interface (see
|
|
sysfs files section below) and can be easily integrated with kdump
|
|
service start/stop init scripts.
|
|
|
|
Comparing with kdump or other strategies, firmware-assisted
|
|
dump offers several strong, practical advantages:
|
|
|
|
- Unlike kdump, the system has been reset, and loaded
|
|
with a fresh copy of the kernel. In particular,
|
|
PCI and I/O devices have been reinitialized and are
|
|
in a clean, consistent state.
|
|
- Once the dump is copied out, the memory that held the dump
|
|
is immediately available to the running kernel. And therefore,
|
|
unlike kdump, FADump doesn't need a 2nd reboot to get back
|
|
the system to the production configuration.
|
|
|
|
The above can only be accomplished by coordination with,
|
|
and assistance from the Power firmware. The procedure is
|
|
as follows:
|
|
|
|
- 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 system crashes, the Power firmware will copy the registered
|
|
low memory regions (boot memory) from source to destination area.
|
|
It will also save hardware PTE's.
|
|
|
|
NOTE:
|
|
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 'crashkernel=' which will override
|
|
the default calculated size. Use this option if default
|
|
boot memory size is not sufficient for second kernel to
|
|
boot successfully. For syntax of crashkernel= parameter,
|
|
refer to Documentation/admin-guide/kdump/kdump.rst. If any
|
|
offset is provided in crashkernel= parameter, it will be
|
|
ignored as FADump uses a predefined offset to reserve memory
|
|
for boot memory dump preservation in case of a crash.
|
|
|
|
- After the low memory (boot memory) area has been saved, the
|
|
firmware will reset PCI and other hardware state. It will
|
|
*not* clear the RAM. It will then launch the bootloader, as
|
|
normal.
|
|
|
|
- The freshly booted kernel will notice that there is a new node
|
|
(rtas/ibm,kernel-dump on pSeries or ibm,opal/dump/mpipl-boot
|
|
on OPAL platform) in the device tree, indicating that
|
|
there is crash data available from a previous boot. During
|
|
the early boot OS will reserve rest of the memory above
|
|
boot memory size effectively booting with restricted memory
|
|
size. This will make sure that this kernel (also, referred
|
|
to as second kernel or capture kernel) will not touch any
|
|
of the dump memory area.
|
|
|
|
- User-space tools will read /proc/vmcore to obtain the contents
|
|
of memory, which holds the previous crashed kernel dump in ELF
|
|
format. The userspace tools may copy this info to disk, or
|
|
network, nas, san, iscsi, etc. as desired.
|
|
|
|
- Once the userspace tool is done saving dump, it will echo
|
|
'1' to /sys/kernel/fadump_release_mem to release the reserved
|
|
memory back to general use, except the memory required for
|
|
next firmware-assisted dump registration.
|
|
|
|
e.g.::
|
|
|
|
# echo 1 > /sys/kernel/fadump_release_mem
|
|
|
|
Please note that the firmware-assisted dump feature
|
|
is only available on POWER6 and above systems on pSeries
|
|
(PowerVM) platform and POWER9 and above systems with OP940
|
|
or later firmware versions on PowerNV (OPAL) platform.
|
|
Note that, OPAL firmware exports ibm,opal/dump node when
|
|
FADump is supported on PowerNV platform.
|
|
|
|
On OPAL based machines, system first boots into an intermittent
|
|
kernel (referred to as petitboot kernel) before booting into the
|
|
capture kernel. This kernel would have minimal kernel and/or
|
|
userspace support to process crash data. Such kernel needs to
|
|
preserve previously crash'ed kernel's memory for the subsequent
|
|
capture kernel boot to process this crash data. Kernel config
|
|
option CONFIG_PRESERVE_FA_DUMP has to be enabled on such kernel
|
|
to ensure that crash data is preserved to process later.
|
|
|
|
-- On OPAL based machines (PowerNV), if the kernel is build with
|
|
CONFIG_OPAL_CORE=y, OPAL memory at the time of crash is also
|
|
exported as /sys/firmware/opal/mpipl/core file. This procfs file is
|
|
helpful in debugging OPAL crashes with GDB. The kernel memory
|
|
used for exporting this procfs file can be released by echo'ing
|
|
'1' to /sys/firmware/opal/mpipl/release_core node.
|
|
|
|
e.g.
|
|
# echo 1 > /sys/firmware/opal/mpipl/release_core
|
|
|
|
Implementation details:
|
|
-----------------------
|
|
|
|
During boot, a check is made to see if firmware supports
|
|
this feature on that particular machine. If it does, then
|
|
we check to see if an active dump is waiting for us. If yes
|
|
then everything but boot memory size of RAM is reserved during
|
|
early boot (See Fig. 2). This area is released once we finish
|
|
collecting the dump from user land scripts (e.g. kdump scripts)
|
|
that are run. If there is dump data, then the
|
|
/sys/kernel/fadump_release_mem file is created, and the reserved
|
|
memory is held.
|
|
|
|
If there is no waiting dump data, then only the memory required to
|
|
hold CPU state, HPTE region, boot memory dump, FADump header and
|
|
elfcore header, is usually reserved at an offset greater than boot
|
|
memory size (see Fig. 1). This area is *not* released: this region
|
|
will be kept permanently reserved, so that it can act as a receptacle
|
|
for a copy of the boot memory content in addition to CPU state and
|
|
HPTE region, in the case a crash does occur.
|
|
|
|
Since this reserved memory area is used only after the system crash,
|
|
there is no point in blocking this significant chunk of memory from
|
|
production kernel. Hence, the implementation uses the Linux kernel's
|
|
Contiguous Memory Allocator (CMA) for memory reservation if CMA is
|
|
configured for kernel. With CMA reservation this memory will be
|
|
available for applications to use it, while kernel is prevented from
|
|
using it. With this FADump will still be able to capture all of the
|
|
kernel memory and most of the user space memory except the user pages
|
|
that were present in CMA region::
|
|
|
|
o Memory Reservation during first kernel
|
|
|
|
Low memory Top of memory
|
|
0 boot memory size |<--- Reserved dump area --->| |
|
|
| | | Permanent Reservation | |
|
|
V V | | V
|
|
+-----------+-----/ /---+---+----+-------+-----+-----+----+--+
|
|
| | |///|////| DUMP | HDR | ELF |////| |
|
|
+-----------+-----/ /---+---+----+-------+-----+-----+----+--+
|
|
| ^ ^ ^ ^ ^
|
|
| | | | | |
|
|
\ CPU HPTE / | |
|
|
------------------------------ | |
|
|
Boot memory content gets transferred | |
|
|
to reserved area by firmware at the | |
|
|
time of crash. | |
|
|
FADump Header |
|
|
(meta area) |
|
|
|
|
|
|
|
|
Metadata: This area holds a metadata structure whose
|
|
address is registered with f/w and retrieved in the
|
|
second kernel after crash, on platforms that support
|
|
tags (OPAL). Having such structure with info needed
|
|
to process the crashdump eases dump capture process.
|
|
|
|
Fig. 1
|
|
|
|
|
|
o Memory Reservation during second kernel after crash
|
|
|
|
Low memory Top of memory
|
|
0 boot memory size |
|
|
| |<------------ Crash preserved area ------------>|
|
|
V V |<--- Reserved dump area --->| |
|
|
+-----------+-----/ /---+---+----+-------+-----+-----+----+--+
|
|
| | |///|////| DUMP | HDR | ELF |////| |
|
|
+-----------+-----/ /---+---+----+-------+-----+-----+----+--+
|
|
| |
|
|
V V
|
|
Used by second /proc/vmcore
|
|
kernel to boot
|
|
|
|
+---+
|
|
|///| -> Regions (CPU, HPTE & Metadata) marked like this in the above
|
|
+---+ figures are not always present. For example, OPAL platform
|
|
does not have CPU & HPTE regions while Metadata region is
|
|
not supported on pSeries currently.
|
|
|
|
Fig. 2
|
|
|
|
|
|
Currently the dump will be copied from /proc/vmcore to a new file upon
|
|
user intervention. The dump data available through /proc/vmcore will be
|
|
in ELF format. Hence the existing kdump infrastructure (kdump scripts)
|
|
to save the dump works fine with minor modifications. KDump scripts on
|
|
major Distro releases have already been modified to work seamlessly (no
|
|
user intervention in saving the dump) when FADump is used, instead of
|
|
KDump, as dump mechanism.
|
|
|
|
The tools to examine the dump will be same as the ones
|
|
used for kdump.
|
|
|
|
How to enable firmware-assisted dump (FADump):
|
|
----------------------------------------------
|
|
|
|
1. Set config option CONFIG_FA_DUMP=y and build kernel.
|
|
2. Boot into linux kernel with 'fadump=on' kernel cmdline option.
|
|
By default, FADump reserved memory will be initialized as CMA area.
|
|
Alternatively, user can boot linux kernel with 'fadump=nocma' to
|
|
prevent FADump to use CMA.
|
|
3. Optionally, user can also set 'crashkernel=' kernel cmdline
|
|
to specify size of the memory to reserve for boot memory dump
|
|
preservation.
|
|
|
|
NOTE:
|
|
1. 'fadump_reserve_mem=' parameter has been deprecated. Instead
|
|
use 'crashkernel=' to specify size of the memory to reserve
|
|
for boot memory dump preservation.
|
|
2. If firmware-assisted dump fails to reserve memory then it
|
|
will fallback to existing kdump mechanism if 'crashkernel='
|
|
option is set at kernel cmdline.
|
|
3. if user wants to capture all of user space memory and ok with
|
|
reserved memory not available to production system, then
|
|
'fadump=nocma' kernel parameter can be used to fallback to
|
|
old behaviour.
|
|
|
|
Sysfs/debugfs files:
|
|
--------------------
|
|
|
|
Firmware-assisted dump feature uses sysfs file system to hold
|
|
the control files and debugfs file to display memory reserved region.
|
|
|
|
Here is the list of files under kernel sysfs:
|
|
|
|
/sys/kernel/fadump_enabled
|
|
This is used to display the FADump status.
|
|
|
|
- 0 = FADump is disabled
|
|
- 1 = FADump is enabled
|
|
|
|
This interface can be used by kdump init scripts to identify if
|
|
FADump is enabled in the kernel and act accordingly.
|
|
|
|
/sys/kernel/fadump_registered
|
|
This is used to display the FADump registration status as well
|
|
as to control (start/stop) the FADump registration.
|
|
|
|
- 0 = FADump is not registered.
|
|
- 1 = FADump is registered and ready to handle system crash.
|
|
|
|
To register FADump echo 1 > /sys/kernel/fadump_registered and
|
|
echo 0 > /sys/kernel/fadump_registered for un-register and stop the
|
|
FADump. Once the FADump is un-registered, the system crash will not
|
|
be handled and vmcore will not be captured. This interface can be
|
|
easily integrated with kdump service start/stop.
|
|
|
|
/sys/kernel/fadump/mem_reserved
|
|
|
|
This is used to display the memory reserved by FADump for saving the
|
|
crash dump.
|
|
|
|
/sys/kernel/fadump_release_mem
|
|
This file is available only when FADump is active during
|
|
second kernel. This is used to release the reserved memory
|
|
region that are held for saving crash dump. To release the
|
|
reserved memory echo 1 to it::
|
|
|
|
echo 1 > /sys/kernel/fadump_release_mem
|
|
|
|
After echo 1, the content of the /sys/kernel/debug/powerpc/fadump_region
|
|
file will change to reflect the new memory reservations.
|
|
|
|
The existing userspace tools (kdump infrastructure) can be easily
|
|
enhanced to use this interface to release the memory reserved for
|
|
dump and continue without 2nd reboot.
|
|
|
|
Note: /sys/kernel/fadump_release_opalcore sysfs has moved to
|
|
/sys/firmware/opal/mpipl/release_core
|
|
|
|
/sys/firmware/opal/mpipl/release_core
|
|
|
|
This file is available only on OPAL based machines when FADump is
|
|
active during capture kernel. This is used to release the memory
|
|
used by the kernel to export /sys/firmware/opal/mpipl/core file. To
|
|
release this memory, echo '1' to it:
|
|
|
|
echo 1 > /sys/firmware/opal/mpipl/release_core
|
|
|
|
Note: The following FADump sysfs files are deprecated.
|
|
|
|
+----------------------------------+--------------------------------+
|
|
| Deprecated | Alternative |
|
|
+----------------------------------+--------------------------------+
|
|
| /sys/kernel/fadump_enabled | /sys/kernel/fadump/enabled |
|
|
+----------------------------------+--------------------------------+
|
|
| /sys/kernel/fadump_registered | /sys/kernel/fadump/registered |
|
|
+----------------------------------+--------------------------------+
|
|
| /sys/kernel/fadump_release_mem | /sys/kernel/fadump/release_mem |
|
|
+----------------------------------+--------------------------------+
|
|
|
|
Here is the list of files under powerpc debugfs:
|
|
(Assuming debugfs is mounted on /sys/kernel/debug directory.)
|
|
|
|
/sys/kernel/debug/powerpc/fadump_region
|
|
This file shows the reserved memory regions if FADump is
|
|
enabled otherwise this file is empty. The output format
|
|
is::
|
|
|
|
<region>: [<start>-<end>] <reserved-size> bytes, Dumped: <dump-size>
|
|
|
|
and for kernel DUMP region is:
|
|
|
|
DUMP: Src: <src-addr>, Dest: <dest-addr>, Size: <size>, Dumped: # bytes
|
|
|
|
e.g.
|
|
Contents when FADump is registered during first kernel::
|
|
|
|
# cat /sys/kernel/debug/powerpc/fadump_region
|
|
CPU : [0x0000006ffb0000-0x0000006fff001f] 0x40020 bytes, Dumped: 0x0
|
|
HPTE: [0x0000006fff0020-0x0000006fff101f] 0x1000 bytes, Dumped: 0x0
|
|
DUMP: [0x0000006fff1020-0x0000007fff101f] 0x10000000 bytes, Dumped: 0x0
|
|
|
|
Contents when FADump is active during second kernel::
|
|
|
|
# cat /sys/kernel/debug/powerpc/fadump_region
|
|
CPU : [0x0000006ffb0000-0x0000006fff001f] 0x40020 bytes, Dumped: 0x40020
|
|
HPTE: [0x0000006fff0020-0x0000006fff101f] 0x1000 bytes, Dumped: 0x1000
|
|
DUMP: [0x0000006fff1020-0x0000007fff101f] 0x10000000 bytes, Dumped: 0x10000000
|
|
: [0x00000010000000-0x0000006ffaffff] 0x5ffb0000 bytes, Dumped: 0x5ffb0000
|
|
|
|
|
|
NOTE:
|
|
Please refer to Documentation/filesystems/debugfs.rst on
|
|
how to mount the debugfs filesystem.
|
|
|
|
|
|
TODO:
|
|
-----
|
|
- Need to come up with the better approach to find out more
|
|
accurate boot memory size that is required for a kernel to
|
|
boot successfully when booted with restricted memory.
|
|
- The FADump implementation introduces a FADump crash info structure
|
|
in the scratch area before the ELF core header. The idea of introducing
|
|
this structure is to pass some important crash info data to the second
|
|
kernel which will help second kernel to populate ELF core header with
|
|
correct data before it gets exported through /proc/vmcore. The current
|
|
design implementation does not address a possibility of introducing
|
|
additional fields (in future) to this structure without affecting
|
|
compatibility. Need to come up with the better approach to address this.
|
|
|
|
The possible approaches are:
|
|
|
|
1. Introduce version field for version tracking, bump up the version
|
|
whenever a new field is added to the structure in future. The version
|
|
field can be used to find out what fields are valid for the current
|
|
version of the structure.
|
|
2. Reserve the area of predefined size (say PAGE_SIZE) for this
|
|
structure and have unused area as reserved (initialized to zero)
|
|
for future field additions.
|
|
|
|
The advantage of approach 1 over 2 is we don't need to reserve extra space.
|
|
|
|
Author: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
|
|
|
|
This document is based on the original documentation written for phyp
|
|
|
|
assisted dump by Linas Vepstas and Manish Ahuja.
|