[PATCH v21 7/7] x86/crash: optimize CPU changes

[Eric DeVolder]

This patch is dependent upon the patch 'crash: change
crash_prepare_elf64_headers() to for_each_possible_cpu()'. With that
patch, crash_prepare_elf64_headers() writes out an ELF CPU PT_NOTE
for all possible CPUs, thus further CPU changes to the elfcorehdr
are not needed.

This change works for kexec_file_load() and kexec_load() syscalls.
For kexec_file_load(), crash_prepare_elf64_headers() is utilized
directly and thus all ELF CPU PT_NOTEs are in the elfcorehdr already.
This is the kimage->file_mode term.
For kexec_load() syscall, one CPU or memory change will cause the
elfcorehdr to be updated via crash_prepare_elf64_headers() and at
that point all ELF CPU PT_NOTEs are in the elfcorehdr. This is the
kimage->elfcorehdr_updated term.

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.

Signed-off-by: Eric DeVolder <eric.devolder@xxxxxxxxxx>
Reviewed-by: Sourabh Jain <sourabhjain@xxxxxxxxxxxxx>
---
 arch/x86/kernel/crash.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
index 0c9d496cf7ce..ead602636f3e 100644
--- a/arch/x86/kernel/crash.c
+++ b/arch/x86/kernel/crash.c
@@ -456,6 +456,16 @@ void arch_crash_handle_hotplug_event(struct kimage *image)
 	unsigned long mem, memsz;
 	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. This works for both kexec_load()
+	 * and kexec_file_load() syscalls.
+	 */
+	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.
-- 
2.31.1