Bug 463645

=Comment: #0=================================================
Emily J. Ratliff <emilyr.com> - 2008-09-16 18:11 EDT
1. Feature Overview:
Feature Id:	[201298]
a. Name of Feature:	CPU Hotplug rewrite
b. Feature Description
With the proliferation of per-cpu data structures in various parts of the kernel, need to handle CPU
hotplug events increased significantly over the years. However, a global mutex became inadequate for
use in many different layers of the kernel some of whom can be used from CPU hotplug event
processing resulting in recursive deadlocks. Recent efforts using per-subsystem locks didn't make
any improvements as potential deadlocks. This rewrite finally converts CPU hotplug to use reference
counting - a more natural model. It uses a reference count (currently global but can be made per-cpu
later) and a waitqueue to protect cpu_online_map. get_online_cpus() and put_online_cpus() are used
to protect critical sections where you may be reading the cpu_online_map. Since this uses a
reference count, you can block between get and put.    This approach has gone through several rounds
of review:  http://lkml.org/lkml/2007/10/24/36  http://lkml.org/lkml/2007/10/16/118.    The last
version of the patch has been added to the sched-devel tree by Ingo and is a merge candidate for
2.6.25:  http://lkml.org/lkml/2007/11/15/239    With this patch, recursive locking problems
(sometimes a 'Lukewarm IQ' message seen with various subsystems will no longer be issues.

Additional Comments:	All the code accepted into 2.6.25

2. Feature Details:
Sponsor:	PPC
Architectures:
x86_64
ppc64

Arch Specificity: Purely Common Code
Affects Core Kernel: Yes
Affects Kernel Modules: Yes
Delivery Mechanism: Direct from community
Category:	Kernel
Request Type:	Kernel - Enhancement from IBM
d. Upstream Acceptance:	Accepted
Sponsor Priority	1
f. Severity: High
IBM Confidential:	no
Code Contribution:	IBM code
g. Component Version Target:	2.6.25

3. Business Case
This work is important for our enterprise customers as part of DLPAR support and Power management
support.

4. Primary contact at Red Hat: 
John Jarvis
jjarvis

5. Primary contacts at Partner:
Project Management Contact:
Michael Hohnbaum, hbaum.com, 503-578-5486

Technical contact(s):
Gautham R Shenoy, gautshen.com
Badari Pulavarty, badari.com