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kexec-kdump-howto.txt: update paragraphs related to disable_cpu_apicid

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Resolves: https://issues.redhat.com/browse/RHEL-127453
Upstream: kdump-utils
Conflict: None

commit e22ca9248de44a71cdd51e0630456e53fe2554c1 (HEAD -> main)
Author: Baoquan He <bhe@redhat.com>
Date:   Thu Dec 11 16:56:15 2025 +0800

    kexec-kdump-howto.txt: update paragraphs related to disable_cpu_apicid

    Long before, to support multiple CPUs on x86_64, 'disable_cpu_apicid='
    was introduced. It's to avoid the case when nr_cpus=xx is added, while
    crashed cpu is not BSP cpu, then the crashed CPU will send INIT to BSP
    cpu in kdump kernel. While the BSP cpu being reinitialized when
    receiving INIT in the 2nd time will cause kdump kernel collapsing.

    Now, in kernel commit 5c5682b9f87a ("x86/cpu: Detect real BSP on crash
    kernels"), the requirement of disable_cpu_apicid has been taken off by
    detecting real BSP on crashed kernel and not sending INIT to it.
    Now testing passed on x86_64 system w/ or w/o disable_cpu_apicid and the
    kernel message can be seen as below.

    ===============
    CPU topo: Boot CPU APIC ID not the first enumerated APIC ID: 1e != 0
    CPU topo: Crash kernel detected. Disabling real BSP to prevent machine INIT
    ===============

    However, sometime older kernels which don't contain commit 5c5682b9f87a
    are still loaded in newer OS. To keep good back compatibility, still
    adding disable_cpu_apicid to capture kernel. It's not harmful on newer
    kernels containing commit 5c5682b9f87a.

    Signed-off-by: Baoquan He <bhe@redhat.com>

Signed-off-by: Baoquan He <bhe@redhat.com>
This commit is contained in:
Baoquan He 2025-12-11 16:56:15 +08:00 committed by Tao Liu
parent b75cbebcfb
commit 2d9d3b40a6
1 changed files with 11 additions and 7 deletions
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18
kexec-kdump-howto.txt
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@ -935,10 +935,14 @@ For example:
Notes on how to use multiple cpus on a capture kernel on x86 system:
Make sure that you are using a kernel that supports disable_cpu_apicid
kernel option as a capture kernel, which is needed to avoid x86 specific
hardware issue (*). The disable_cpu_apicid kernel option is automatically
appended by kdumpctl script and is ignored if the kernel doesn't support it.
The disable_cpu_apicid kernel option is automatically appended by kdumpctl
script to capture kernel to avoid x86 specific hardware issue and is ignored
if the kernel doesn't support it. For newer kernels that contain commit
5c5682b9f87a ("x86/cpu: Detect real BSP on crash kernels"), this hardware
issue on x86 has been fixed in the kernel. However, adding disable_cpu_apicid
is not harmful for newer kernels, and it is still added to the capture kernel
to maintain backward compatibility with older kernels that do not contain
commit 5c5682b9f87a.
You need to specify how many cpus to be used in a capture kernel by specifying
the number of cpus in nr_cpus kernel option in /etc/sysconfig/kdump. nr_cpus
@ -948,9 +952,9 @@ You should use necessary and sufficient number of cpus on a capture kernel.
Warning: Don't use too many cpus on a capture kernel, or the capture kernel
may lead to panic due to Out Of Memory.
(*) Without disable_cpu_apicid kernel option, capture kernel may lead to
hang, system reset or power-off at boot, depending on your system and runtime
situation at the time of crash.
(*) Without the disable_cpu_apicid kernel option, a capture kernel that does
not contain commit 5c5682b9f87a may hang, reset, or power-off at boot,
depending on your system and runtime situation at the time of the crash.
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