1. Computers
  2. Display Drivers
  3. Graphics Cards
  4. Memory
  5. Motherboards
  6. Processors
  7. Software
  8. Storage
  9. Operating Systems


Facebook RSS Twitter Twitter Google Plus


Phoronix Test Suite

OpenBenchmarking.org

New Linux CPU Hot-Plugging Works Out "Nightmare"

Linux Kernel

Published on 31 January 2013 03:25 PM EST
Written by Michael Larabel in Linux Kernel
13 Comments

The current Linux kernel CPU hot-plugging support has been described as "an increasing nightmare full of races and undocumented behaviour", but fortunately it's in the process of being re-developed.

Thomas Gleixner has been one of the kernel developers looking to rework the Linux CPU hot-plug support and published a new patch-set today. Hot-plugging support in general has been a focus lately with work on a common system device hot-plug framework for the kernel, true CPU hot-plug support, ACPI hot-plug improvements, and other efforts in recent months.

Gleixner's CPU hot-plug re-work that was published today consists of 40 patches that amount to over one thousand lines of changed code within the kernel. Below is his description of the massive CPU hot-plug changes for the Linux kernel.
The current CPU hotplug implementation has become an increasing nightmare full of races and undocumented behaviour. The main issue of the current hotplug scheme is the completely asymetric startup/teardown process. The hotplug notifiers are mostly undocumented and the CPU_* actions in lots of implementations seem to be randomly chosen.

We had a long discussion in San Diego last year about reworking the hotplug core into a fully symetric state machine. After a few doomed attempts to convert the existing code into a state machine, I finally found a workable solution.

The following patch series implements a trivial array based state machine, which replaces the existing steps in cpu_up/down and also the notifiers which must run on the hotplugged cpu are converted to a callback array. This documents clearly the ordering of the callbacks and also makes the asymetric behaviour very obvious.

This series converts the stop_machine thread to the smpboot infrastructure, implements the core state machine and converts all notifiers which have ordering constraints plus a randomly chosen bunch of other notifiers to the state machine.

The runtime installed callbacks are immediately executed by the core code on or on behalf of all cpus which have already reached the corresponding state. A non executing installer function is there as well to allow simple migration of the existing notifier maze.

The diffstat of the complete series is appended below.

36 files changed, 1300 insertions(+), 1179 deletions(-)

We add slightly more code at this stage (225 lines alone in a header file), but most of the conversions are removing code and we have only tackled about 30 of 130+ instances. Even with the current conversion state, the resulting text size shrinks already.
The current patch-set can be found in CPU hotplug rework - episode I.

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 .
Latest Linux Hardware Reviews
  1. 13-Way Low-End GPU Comparison With AMD's AM1 Athlon
  2. ASUS AM1I-A: A Mini-ITX Board For Socketed Kabini APUs
  3. Mini-Box M350: A Simple, Affordable Mini-ITX Case
  4. Overclocking The AMD AM1 Athlon & Sempron APUs
Latest Linux Articles
  1. Ubuntu 12.04.4 vs. 13.10 vs. 14.04 LTS Desktop Benchmarks
  2. AMD OpenCL Performance With AM1 Kabini APUs
  3. A Quick Look At GCC 4.9 vs. LLVM Clang 3.5
  4. Are AMD Athlon/Sempron APUs Fast Enough For Steam On Linux?
Latest Linux News
  1. Mesa 10.0 & 10.1 Stable Get Updated
  2. Getting Hit By The Variable Performance Of The Public Cloud
  3. Git 2.0 Test Releases Begin With Many Changes
  4. Wine 1.7.17 Works On Its Task Scheduler, C Run-Time
  5. The Improv ARM Board Still Isn't Shipping; Riding A Dead Horse?
  6. Debian To Maintain 6.0 Squeeze As An LTS Release
  7. Wasteland 2 Is Finally Released For Linux Gamers
  8. FreeBSD Advances For ARM, Bhyve, Clang
  9. Ubuntu 14.04 LTS "Trusty Tahr" Officially Released
  10. Ubuntu 12.04 LTS vs. 14.04 LTS Server Benchmarks
  11. QEMU 2.0 Released With ARM, x86 Enhancements
  12. Running The Unity 8 Preview Session On Ubuntu 14.04 LTS
Latest Forum Discussions
  1. Suggestions about how to make a Radeon HD 7790 work decently?
  2. The GNOME Foundation Is Running Short On Money
  3. Updated and Optimized Ubuntu Free Graphics Drivers
  4. Radeon 8000M problematic on Linux?
  5. Linux Kernel Developers Fed Up With Ridiculous Bugs In Systemd
  6. After Jack Keane, RuseSoft will briing Ankh 3 to Linux through Desura
  7. Suspected PHP Proxy Issue
  8. Change installation destination from home directory