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    SubjectRe: [PATCH v19 7/7] x86/crash: optimize cpu changes
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    On 06/03/23 21:52, Eric DeVolder wrote:
    > 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@oracle.com>
    > ---
    > 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 c9aaec9de775..82ea2b1bdc61 100644
    > --- a/arch/x86/kernel/crash.c
    > +++ b/arch/x86/kernel/crash.c
    > @@ -471,6 +471,16 @@ 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;

    I like the idea of having elfcorehdr_updated to track elfcoredhr segment
    updates.
    It handles the possible CPU thing for kexec_load syscall nicely.

    Thanks for incorporating the possible CPU change.

    The patch series looks good to me.
    Reviewed-by: Sourabh Jain <sourabhjain@linux.ibm.com>

    - Sourabh Jain

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