[PATCH v5] reboot: support offline CPUs before reboot

[Hsin-Yi Wang]

On Wed, Jan 15, 2020 at 7:41 PM Sudeep Holla <sudeep.holla at arm.com> wrote:
>
> On Wed, Jan 15, 2020 at 02:34:10PM +0800, Hsin-Yi Wang wrote:
> > Currently system reboots uses architecture specific codes (smp_send_stop)
> > to offline non reboot CPUs. Most architecture's implementation is looping
> > through all non reboot online CPUs and call ipi function to each of them. Some
> > architecture like arm64, arm, and x86... would set offline masks to cpu without
> > really offline them. This causes some race condition and kernel warning comes
> > out sometimes when system reboots.
> >
> > This patch adds a config ARCH_OFFLINE_CPUS_ON_REBOOT, which would offline cpus in
> > migrate_to_reboot_cpu(). If non reboot cpus are all offlined here, the loop for
> > checking online cpus would be an empty loop. If architecture don't enable this
> > config, or some cpus somehow fails to offline, it would fallback to ipi
> > function.
> >
>
> What's the timing impact on systems with large number of CPUs(say 256 or
> more) ? I remember we added some change to reduce the wait times for
> offlining CPUs in system suspend path on arm64, still not negligible.
>

This is not the final solution, but I would still provided some data
points here:

Tested on my arm64 with 4 cpu: 2 a53 and 2 a72.
Offlining 3 cpu takes about 60~65 ms
Offlining 2 cpu(a53+a72 or a72+a72) takes about 42~47 ms
Offlining 1 cpu(a53 or a72) takes about 23~25 ms.

It would take longer time for systems with large number of CPUs.