Re: [PATCH v14 5/6] powerpc: add crash CPU hotplug support
Hi Sourabh
On 11/12/23 2:00 pm, Sourabh Jain wrote:
Due to CPU/Memory hotplug or online/offline events the elfcorehdr
(which describes the CPUs and memory of the crashed kernel) and FDT
(Flattened Device Tree) of kdump image becomes outdated. Consequently,
attempting dump collection with an outdated elfcorehdr or FDT can lead
to failed or inaccurate dump collection.
Going forward CPU hotplug or online/offlice events are referred as
s/offlice/offline/
CPU/Memory add/remvoe events.
s/remvoe/remove/
The current solution to address the above issue involves monitoring the
CPU/memory add/remove events in userspace using udev rules and whenever
there are changes in CPU and memory resources, the entire kdump image
is loaded again. The kdump image includes kernel, initrd, elfcorehdr,
FDT, purgatory. Given that only elfcorehdr and FDT get outdated due to
CPU/Memory add/remove events, reloading the entire kdump image is
inefficient. More importantly, kdump remains inactive for a substantial
amount of time until the kdump reload completes.
To address the aforementioned issue, commit 247262756121 ("crash: add
generic infrastructure for crash hotplug support") added a generic
infrastructure that allows architectures to selectively update the kdump
image component during CPU or memory add/remove events within the kernel
itself.
In the event of a CPU or memory add/remove event, the generic crash
hotplug event handler, `crash_handle_hotplug_event()`, is triggered. It
then acquires the necessary locks to update the kdump image and invokes
the architecture-specific crash hotplug handler,
`arch_crash_handle_hotplug_event()`, to update the required kdump image
components.
This patch adds crash hotplug handler for PowerPC and enable support to
update the kdump image on CPU add/remove events. Support for memory
add/remove events is added in a subsequent patch with the title
"powerpc: add crash memory hotplug support."
As mentioned earlier, only the elfcorehdr and FDT kdump image components
need to be updated in the event of CPU or memory add/remove events.
However, the PowerPC architecture crash hotplug handler only updates the
FDT to enable crash hotplug support for CPU add/remove events. Here's
why.
The Elfcorehdr on PowerPC is built with possible CPUs, and thus, it does
not need an update on CPU add/remove events. On the other hand, the FDT
needs to be updated on CPU add events to include the newly added CPU. If
the FDT is not updated and the kernel crashes on a newly added CPU, the
kdump kernel will fail to boot due to the unavailability of the crashing
CPU in the FDT. During the early boot, it is expected that the boot CPU
must be a part of the FDT; otherwise, the kernel will raise a BUG and
fail to boot. For more information, refer to commit 36ae37e3436b0
("powerpc: Make boot_cpuid common between 32 and 64-bit"). Since it is
okay to have an offline CPU in the kdump FDT, no action is taken in case
of CPU removal.
There are two system calls, `kexec_file_load` and `kexec_load`, used to
load the kdump image. Few changes have been made to ensure kernel can
safely update the kdump FDT for both system calls.
For kexec_file_load syscall the kdump image is prepared in kernel. So to
support an increasing number of CPUs, the FDT is constructed with extra
buffer space to ensure it can accommodate a possible number of CPU
nodes. Additionally, a call to fdt_pack (which trims the unused space
once the FDT is prepared) is avoided for kdump image loading if this
feature is enabled.
For the kexec_load syscall, the FDT is updated only if both the
KEXEC_UPDATE_FDT and KEXEC_UPDATE_ELFCOREHDR kexec flags are passed to
the kernel by the kexec tool. Passing these flags to the kernel
indicates that the FDT is built to accommodate possible CPUs, and the
FDT segment is not considered for SHA calculation, making it safe to
update the FDT.
Commit 88a6f8994421 ("crash: memory and CPU hotplug sysfs attributes")
added a sysfs interface to indicate userspace (kdump udev rule) that
kernel will update the kdump image on CPU hotplug events, so kdump
reload can be avoided. Implement arch specific function
`arch_crash_hotplug_cpu_support()` to correctly advertise kernel
capability to update kdump image.
This feature is advertised to userspace when the following conditions
are met:
1. Kdump image is loaded using kexec_file_load system call.
2. Kdump image is loaded using kexec_load system and both
KEXEC_UPATE_ELFCOREHDR and KEXEC_UPDATE_FDT kexec flags are
passed to kernel.
The changes related to this feature are kept under the CRASH_HOTPLUG
config, and it is enabled by default.
Signed-off-by: Sourabh Jain
Cc: Akhil Raj
Cc: Andrew Morton
Cc: Aneesh Kumar K.V
Cc: Baoquan He
Cc: Borislav Petkov (AMD)
Cc: Boris Ostrovsky
Cc: Christophe Leroy
Cc: Dave Hansen
Cc: Dave Young
Cc: David Hildenbrand
Cc: Eric DeVolder
Cc: Greg Kroah-Hartman
Cc: Hari Bathini
Cc: Laurent Dufour
Cc: Mahesh Salgaonkar
Cc: Michael Ellerman
[PATCH v14 5/6] powerpc: add crash CPU hotplug support
Due to CPU/Memory hotplug or online/offline events the elfcorehdr
(which describes the CPUs and memory of the crashed kernel) and FDT
(Flattened Device Tree) of kdump image becomes outdated. Consequently,
attempting dump collection with an outdated elfcorehdr or FDT can lead
to failed or inaccurate dump collection.
Going forward CPU hotplug or online/offlice events are referred as
CPU/Memory add/remvoe events.
The current solution to address the above issue involves monitoring the
CPU/memory add/remove events in userspace using udev rules and whenever
there are changes in CPU and memory resources, the entire kdump image
is loaded again. The kdump image includes kernel, initrd, elfcorehdr,
FDT, purgatory. Given that only elfcorehdr and FDT get outdated due to
CPU/Memory add/remove events, reloading the entire kdump image is
inefficient. More importantly, kdump remains inactive for a substantial
amount of time until the kdump reload completes.
To address the aforementioned issue, commit 247262756121 ("crash: add
generic infrastructure for crash hotplug support") added a generic
infrastructure that allows architectures to selectively update the kdump
image component during CPU or memory add/remove events within the kernel
itself.
In the event of a CPU or memory add/remove event, the generic crash
hotplug event handler, `crash_handle_hotplug_event()`, is triggered. It
then acquires the necessary locks to update the kdump image and invokes
the architecture-specific crash hotplug handler,
`arch_crash_handle_hotplug_event()`, to update the required kdump image
components.
This patch adds crash hotplug handler for PowerPC and enable support to
update the kdump image on CPU add/remove events. Support for memory
add/remove events is added in a subsequent patch with the title
"powerpc: add crash memory hotplug support."
As mentioned earlier, only the elfcorehdr and FDT kdump image components
need to be updated in the event of CPU or memory add/remove events.
However, the PowerPC architecture crash hotplug handler only updates the
FDT to enable crash hotplug support for CPU add/remove events. Here's
why.
The Elfcorehdr on PowerPC is built with possible CPUs, and thus, it does
not need an update on CPU add/remove events. On the other hand, the FDT
needs to be updated on CPU add events to include the newly added CPU. If
the FDT is not updated and the kernel crashes on a newly added CPU, the
kdump kernel will fail to boot due to the unavailability of the crashing
CPU in the FDT. During the early boot, it is expected that the boot CPU
must be a part of the FDT; otherwise, the kernel will raise a BUG and
fail to boot. For more information, refer to commit 36ae37e3436b0
("powerpc: Make boot_cpuid common between 32 and 64-bit"). Since it is
okay to have an offline CPU in the kdump FDT, no action is taken in case
of CPU removal.
There are two system calls, `kexec_file_load` and `kexec_load`, used to
load the kdump image. Few changes have been made to ensure kernel can
safely update the kdump FDT for both system calls.
For kexec_file_load syscall the kdump image is prepared in kernel. So to
support an increasing number of CPUs, the FDT is constructed with extra
buffer space to ensure it can accommodate a possible number of CPU
nodes. Additionally, a call to fdt_pack (which trims the unused space
once the FDT is prepared) is avoided for kdump image loading if this
feature is enabled.
For the kexec_load syscall, the FDT is updated only if both the
KEXEC_UPDATE_FDT and KEXEC_UPDATE_ELFCOREHDR kexec flags are passed to
the kernel by the kexec tool. Passing these flags to the kernel
indicates that the FDT is built to accommodate possible CPUs, and the
FDT segment is not considered for SHA calculation, making it safe to
update the FDT.
Commit 88a6f8994421 ("crash: memory and CPU hotplug sysfs attributes")
added a sysfs interface to indicate userspace (kdump udev rule) that
kernel will update the kdump image on CPU hotplug events, so kdump
reload can be avoided. Implement arch specific function
`arch_crash_hotplug_cpu_support()` to correctly advertise kernel
capability to update kdump image.
This feature is advertised to userspace when the following conditions
are met:
1. Kdump image is loaded using kexec_file_load system call.
2. Kdump image is loaded using kexec_load system and both
KEXEC_UPATE_ELFCOREHDR and KEXEC_UPDATE_FDT kexec flags are
passed to kernel.
The changes related to this feature are kept under the CRASH_HOTPLUG
config, and it is enabled by default.
Signed-off-by: Sourabh Jain
Cc: Akhil Raj
Cc: Andrew Morton
Cc: Aneesh Kumar K.V
Cc: Baoquan He
Cc: Borislav Petkov (AMD)
Cc: Boris Ostrovsky
Cc: Christophe Leroy
Cc: Dave Hansen
Cc: Dave Young
Cc: David Hildenbrand
Cc: Eric DeVolder
Cc: Greg Kroah-Hartman
Cc: Hari Bathini
Cc: Laurent Dufour
Cc: Mahesh Salgaonkar
Cc: Michael Ellerman
Cc: Mimi Zohar
Cc: Naveen N Rao
Cc: Oscar Salvador
Cc: Thomas Gleixner
Cc: Valentin Schneider
