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Linux Works Towards True CPU Hotplug Support

Linux Kernel

Published on 11 December 2012 11:28 AM EST
Written by Michael Larabel in Linux Kernel
15 Comments

One of the first pull requests sent in following the the Linux 3.7 kernel release is for getting closer to "true CPU hotplug support" with the Linux 3.8 kernel.

There isn't yet "true" CPU hotplug support for Linux 3.8, but developers are getting closer. Ingo Molnar sent in this morning the x86 generic pull request for 3.8. While the Linux kernel has had CPU hotplugging support, up to this point you haven't been able to hotplug the first (boot) processor.

With this pull for the Linux 3.8 kernel, it will become possible to online/offline the boot processor just like any other x86 CPUs. This feature right now will be supported first by Intel CPUs. For 3.8, this feature is behind a configuration option that is disabled by default.
This tree enables CPU#0 (the boot processor) to be onlined/offlined on x86, just like any other CPU. Enabled on Intel CPUs for now.

Allowing this required the identification and fixing of latent CPU#0 assumptions (such as CPU#0 initializations, etc.) in the x86 architecture code, plus the identification of barriers to BSP-offlining, such as active PIC interrupts which can only be serviced on the BSP.

It's behind a default-off option, and there's a debug option that allows the automatic testing of this feature.

The motivation of this feature is to allow and prepare for true CPU-hotplug hardware support: recent changes to MCE support enable us to detect a deteriorating but not yet hard-failing L1/L2 cache on a CPU that could be soft-unplugged - or a failing L3 cache on a multi-socket system.

Note that true hardware hot-plug is not yet fully enabled by this, because that requires a special platform wakeup sequence to be sent to the freshly powered up CPU#0. Future patches for this are planned, once such a platform exists. Chicken and egg

About The Author
Michael Larabel is the principal author of Phoronix.com and founded the web-site in 2004 with a focus on enriching the Linux hardware experience and being the largest web-site devoted to Linux hardware reviews, particularly for products relevant to Linux gamers and enthusiasts but also commonly reviewing servers/workstations and embedded Linux devices. Michael has written more than 10,000 articles covering the state of Linux hardware support, Linux performance, graphics hardware drivers, and other topics. Michael is also the lead developer of the Phoronix Test Suite, Phoromatic, and OpenBenchmarking.org automated testing software. He can be followed via and or contacted via .
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