On 09/07/2019 20:20, Pavel Tatashin wrote: > Changelog > v1 - v2 > - No changes to patches, addressed suggestion from James Morse > to add "arm64" tag to cover letter. > - Improved cover letter information based on discussion. > > Currently, it is only allowed to reserve memory for crash kernel, because > it is a requirement in order to be able to boot into crash kernel without > touching memory of crashed kernel is to have memory reserved. > > The second benefit for having memory reserved for kexec kernel is > that it does not require a relocation after segments are loaded into > memory. > > If kexec functionality is used for a fast system update, with a minimal > downtime, the relocation of kernel + initramfs might take a significant > portion of reboot. > > In fact, on the machine that we are using, that has ARM64 processor > it takes 0.35s to relocate during kexec, thus taking 52% of kernel reboot > time: > > kernel shutdown 0.03s > relocation 0.35s > kernel startup 0.29s > > Image: 13M and initramfs is 24M. If initramfs increases, the relocation > time increases proportionally. > > While, it is possible to add 'kexeckernel=' parameters support to other > architectures by modifying reserve_crashkernel(), in this series this is > done for arm64 only. > I wonder if we couldn't use the crashkernel reserved memory area for that and just add logic to kexec-tools to pass to the kernel a flag (a new magic reboot number?) to use the crashkernel memory for that? The kernel would then unload the crash/capture system in the reserved memory area and reuse the latter for kexec. This would also enable the feature for all architectures. Regards, Matthias > The reason it is so slow on arm64 to relocate kernel is because the code > that does relocation does this with MMU disabled, and thus D-Cache and > I-Cache must also be disabled. > > Alternative solution is more complicated: Setup a temporary page table > for relocation_routine and also for code from cpu_soft_restart. Perform > relocation with MMU enabled, do cpu_soft_restart where MMU and caching > are disabled, jump to purgatory. A similar approach was suggested for > purgatory and was rejected due to making purgatory too complicated. > On, the other hand hibernate does something similar already, but there > MMU never needs to be disabled, and also by the time machine_kexec() > is called, allocator is not available, as we can't fail to do reboot, > so page table must be pre-allocated during kernel load time. > > Note: the above time is relocation time only. Purgatory usually also > computes checksum, but that is skipped, because --no-check is used when > kernel image is loaded via kexec. > > Pavel Tatashin (5): > kexec: quiet down kexec reboot > kexec: add resource for normal kexec region > kexec: export common crashkernel/kexeckernel parser > kexec: use reserved memory for normal kexec reboot > arm64, kexec: reserve kexeckernel region > > .../admin-guide/kernel-parameters.txt | 7 ++ > arch/arm64/kernel/setup.c | 5 ++ > arch/arm64/mm/init.c | 83 ++++++++++++------- > include/linux/crash_core.h | 6 ++ > include/linux/ioport.h | 1 + > include/linux/kexec.h | 6 +- > kernel/crash_core.c | 27 +++--- > kernel/kexec_core.c | 50 +++++++---- > 8 files changed, 127 insertions(+), 58 deletions(-) >
