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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.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.