Re: [v1 0/5] allow to reserve memory for normal kexec kernel

[Pavel Tatashin]

On Tue, Jul 9, 2019 at 6:36 AM Bhupesh Sharma <bhsharma@xxxxxxxxxx> wrote:
>
> Hi Pavel,
>
> On Tue, Jul 9, 2019 at 2:46 AM Pavel Tatashin <pasha.tatashin@xxxxxxxxxx> wrote:
> >
> > 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.
> >
> > 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(-)
> >
> > --
> > 2.22.0
>
> This seems like an issue with time spent while doing sha256
> verification while in purgatory.
>
> Can you please try the following two patches which enable D-cache in
> purgatory before SHA verification and disable it before switching to
> kernel:
>
> http://lists.infradead.org/pipermail/kexec/2017-May/018839.html
> http://lists.infradead.org/pipermail/kexec/2017-May/018840.html

Hi Bhupesh,

The verification was taking 2.31s. This is why it is disabled via
kexec's '-i' flag. Therefore 0.35s is only the relocation part where
time is spent, and with my patches the time is completely gone.
Actually, I am glad you showed these patches to me because I might
pull them and enable verification for our needs.

>
> Note that these were not accepted upstream but are included in several
> distros in some form or the other :)

Enabling MMU and D-Cache for relocation  would essentially require the
same changes in kernel. Could you please share exactly why these were
not accepted upstream into kexec-tools?

Thank you,
Pasha

>
> Thanks,
> Bhupesh