+ x86-crash-optimize-cpu-changes.patch added to mm-nonmm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

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The patch titled
     Subject: x86/crash: optimize CPU changes
has been added to the -mm mm-nonmm-unstable branch.  Its filename is
     x86-crash-optimize-cpu-changes.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/x86-crash-optimize-cpu-changes.patch

This patch will later appear in the mm-nonmm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Eric DeVolder <eric.devolder@oracle.com>
Subject: x86/crash: optimize CPU changes
Date: Fri, 11 Aug 2023 13:06:42 -0400

crash_prepare_elf64_headers() writes into the elfcorehdr an ELF PT_NOTE
for all possible CPUs.  As such, subsequent changes to CPUs (ie.  hot
un/plug, online/offline) do not need to rewrite the elfcorehdr.

The kimage->file_mode term covers kdump images loaded via the
kexec_file_load() syscall.  Since crash_prepare_elf64_headers() wrote the
initial elfcorehdr, no update to the elfcorehdr is needed for CPU changes.

The kimage->elfcorehdr_updated term covers kdump images loaded via the
kexec_load() syscall.  At least one memory or CPU change must occur to
cause crash_prepare_elf64_headers() to rewrite the elfcorehdr. 
Afterwards, no update to the elfcorehdr is needed for CPU changes.

This code is intentionally *NOT* hoisted into crash_handle_hotplug_event()
as it would prevent the arch-specific handler from running for CPU
changes.  This would break PPC, for example, which needs to update other
information besides the elfcorehdr, on CPU changes.

Link: https://lkml.kernel.org/r/20230811170642.6696-9-eric.devolder@oracle.com
Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
Reviewed-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Acked-by: Hari Bathini <hbathini@linux.ibm.com>
Acked-by: Baoquan He <bhe@redhat.com>
Cc: Akhil Raj <lf32.dev@gmail.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Borislav Petkov (AMD) <bp@alien8.de>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Mimi Zohar <zohar@linux.ibm.com>
Cc: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Takashi Iwai <tiwai@suse.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Thomas Weißschuh <linux@weissschuh.net>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 arch/x86/kernel/crash.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

--- a/arch/x86/kernel/crash.c~x86-crash-optimize-cpu-changes
+++ a/arch/x86/kernel/crash.c
@@ -468,6 +468,16 @@ void arch_crash_handle_hotplug_event(str
 	unsigned long elfsz = 0;
 
 	/*
+	 * As crash_prepare_elf64_headers() has already described all
+	 * possible CPUs, there is no need to update the elfcorehdr
+	 * for additional CPU changes.
+	 */
+	if ((image->file_mode || image->elfcorehdr_updated) &&
+		((image->hp_action == KEXEC_CRASH_HP_ADD_CPU) ||
+		(image->hp_action == KEXEC_CRASH_HP_REMOVE_CPU)))
+		return;
+
+	/*
 	 * Create the new elfcorehdr reflecting the changes to CPU and/or
 	 * memory resources.
 	 */
_

Patches currently in -mm which might be from eric.devolder@oracle.com are

kexec-consolidate-kexec-and-crash-options-into-kernel-kconfigkexec.patch
x86-kexec-refactor-for-kernel-kconfigkexec.patch
arm-kexec-refactor-for-kernel-kconfigkexec.patch
ia64-kexec-refactor-for-kernel-kconfigkexec.patch
arm64-kexec-refactor-for-kernel-kconfigkexec.patch
loongarch-kexec-refactor-for-kernel-kconfigkexec.patch
m68k-kexec-refactor-for-kernel-kconfigkexec.patch
mips-kexec-refactor-for-kernel-kconfigkexec.patch
parisc-kexec-refactor-for-kernel-kconfigkexec.patch
powerpc-kexec-refactor-for-kernel-kconfigkexec.patch
riscv-kexec-refactor-for-kernel-kconfigkexec.patch
s390-kexec-refactor-for-kernel-kconfigkexec.patch
sh-kexec-refactor-for-kernel-kconfigkexec.patch
kexec-rename-arch_has_kexec_purgatory.patch
remove-arch_default_kexec-from-kconfigkexec.patch
crash-move-a-few-code-bits-to-setup-support-of-crash-hotplug.patch
crash-add-generic-infrastructure-for-crash-hotplug-support.patch
kexec-exclude-elfcorehdr-from-the-segment-digest.patch
crash-memory-and-cpu-hotplug-sysfs-attributes.patch
x86-crash-add-x86-crash-hotplug-support.patch
crash-hotplug-support-for-kexec_load.patch
crash-change-crash_prepare_elf64_headers-to-for_each_possible_cpu.patch
x86-crash-optimize-cpu-changes.patch