Booting the kernel with "maxcpus=1" is a common technique for CPU
partitioning and isolation. It delays the CPU bringup process until
when the bootup scripts are ready to bring CPUs online by writing 1 to
/sys/device/system/cpu/cpu<X>/online. However, it was found that not
all the CPUs were online after bootup. The collection of offline CPUs
are different after every reboot.

Further investigation reveals that some "online" write operations
fail with an -EBUSY error. This error is returned when CPU hotplug is
temporiarly disabled when cpu_hotplug_disable() is called.

During bootup, the main caller of cpu_hotplug_disable() is
pci_call_probe() for PCI device initialization. By measuring the times
spent with cpu_hotplug_disabled set in a typical 2-socket server, most
of them last less than 10ms.  However, there are a few that can last
hundreds of ms. Note that the cpu_hotplug_disabled period of different
devices can overlap leading to longer cpu_hotplug_disabled hold time.

Since the CPU hotplug disable condition is transient and it is not
that easy to modify all the existing bootup scripts to handle this
condition, the kernel can help by retrying the online operation when
an -EBUSY error is returned. This patch retries the online operation
in cpu_subsys_online() when an -EBUSY error is returned for up to 5
times after an exponentially increasing delay that can last a total of
at least 620ms of waiting time by calling msleep().

With this patch in place, booting up the patched kernel with "maxcpus=1"
does not leave any CPU in an offline state in 10 reboot attempts.
Reported-by: Vishal Agrawal <vagrawal@redhat.com>
Signed-off-by: Waiman Long <longman@redhat.com>
Link: https://lore.kernel.org/r/20230724143826.3996163-1-longman@redhat.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>