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.

--
Regards,
Sudeep

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