Bug 712653

Summary:	make guest mode entry to be rcu quiescent state
Product:	Red Hat Enterprise Linux 6	Reporter:	Gleb Natapov <gleb>
Component:	kernel	Assignee:	Gleb Natapov <gleb>
Status:	CLOSED ERRATA	QA Contact:	Virtualization Bugs <virt-bugs>
Severity:	high	Docs Contact:
Priority:	urgent
Version:	6.2	CC:	cww, dhoward, juzhang, knoel, skito, tburke, tmuneda
Target Milestone:	rc	Keywords:	ZStream
Target Release:	---
Hardware:	All
OS:	Linux
Whiteboard:
Fixed In Version:	kernel-2.6.32-160.el6	Doc Type:	Bug Fix
Doc Text:	When a CPU is about to modify data protected by the RCU (Read Copy Update) mechanism, it has to wait for other CPUs in the system to pass a quiescent state. Previously, the guest mode was not considered a quiescent state. As a consequence, if a CPU was in the guest mode for a long time, another CPU had to wait a long time in order to modify RCU-protected data. With this update, the rcu_virt_note_context_switch() function, which marks the guest mode as a quiescent state, has been added to the kernel, thus resolving this issue.	Story Points:	---
Clone Of:		Environment:
Last Closed:	2011-12-06 13:38:43 UTC	Type:	---
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:
Bug Depends On:
Bug Blocks:	740352

Description Gleb Natapov 2011-06-12 07:50:39 UTC

KVM does not hold any references to rcu protected data when it switches
CPU into a guest mode. In fact switching to a guest mode is very similar
to exiting to userspase from rcu point of view. In addition CPU may stay
in a guest mode for quite a long time (up to one time slice). Lets treat
guest mode as quiescent state, just like we do with user-mode execution.

Comment 2 RHEL Program Management 2011-06-13 16:10:21 UTC

This request was evaluated by Red Hat Product Management for inclusion
in a Red Hat Enterprise Linux maintenance release. Product Management has 
requested further review of this request by Red Hat Engineering, for potential
inclusion in a Red Hat Enterprise Linux Update release for currently deployed 
products. This request is not yet committed for inclusion in an Update release.

Comment 3 juzhang 2011-06-16 07:51:33 UTC

Tested on 2.6.32-156.el6.x86_64 with qemu-kvm-0.12.1.2-2.165.el6.x86_64, marked as qa_ack+

steps
1. boot guest with virtio devices and vcpu=4
2. in guest
#halt

Result:
guest just pause for a couple of seconds after printing "Disabling non-boot CPUs"

Please note:
guest should be rhel6.X

Comment 4 Aristeu Rozanski 2011-06-27 19:04:43 UTC

Patch(es) available on kernel-2.6.32-160.el6

Comment 10 juzhang 2011-10-13 05:18:13 UTC

Verified this issue on kernel 2.6.32-206.el6.x86_64 with qemu-kvm-0.12.1.2-2.195.el6.x86_64.,I tried three times.

1.boot guest with boot guest with virtio devices and vcpu=4
2.in guest
#half

Results:
No pause after printing "Disabling non-boot
CPUs"




Guest kernel
2.6.32-206.el6.x86_64

Comment 11 juzhang 2011-10-13 05:19:02 UTC

According to comment3 and comment10,set this issue as verified.

Comment 12 Tomas Capek 2011-11-23 15:25:58 UTC

    Technical note added. If any revisions are required, please edit the "Technical Notes" field
    accordingly. All revisions will be proofread by the Engineering Content Services team.
    
    New Contents:
When a CPU is about to modify data protected by the RCU (Read Copy Update) mechanism, it has to wait for other CPUs in the system to pass a quiescent state. Previously, the guest mode was not considered a quiescent state. As a consequence, if a CPU was in the guest mode for a long time, another CPU had to wait a long time in order to modify RCU-protected data. With this update, the rcu_virt_note_context_switch() function, which marks the guest mode as a quiescent state, has been added to the kernel, thus resolving this issue.

Comment 13 errata-xmlrpc 2011-12-06 13:38:43 UTC

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

http://rhn.redhat.com/errata/RHSA-2011-1530.html