Bug 1344077

Summary: turn CONFIG_RCU_NOCB_CPU_ALL=y off
Product: Red Hat Enterprise Linux 7 Reporter: Daniel Bristot de Oliveira <daolivei>
Component: kernel-rtAssignee: Clark Williams <williams>
kernel-rt sub component: Other QA Contact: Jiri Kastner <jkastner>
Status: CLOSED ERRATA Docs Contact:
Severity: high    
Priority: medium CC: bhu, jshortt
Version: 7.3Keywords: ZStream
Target Milestone: rc   
Target Release: ---   
Hardware: x86_64   
OS: Linux   
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Clone Of:
: 1348715 1349770 (view as bug list) Environment:
Last Closed: 2016-11-03 19:52:58 UTC Type: Bug
Regression: --- Mount Type: ---
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: --- Target Upstream Version:
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Bug Depends On:    
Bug Blocks: 1274397, 1348715, 1349770    

Description Daniel Bristot de Oliveira 2016-06-08 17:02:02 UTC 
Description of problem:

Although rcu no callbacks is useful for the majority of -rt workloads. There are cases where this feature can cause bad impact in the -rt performance.

For example, if the rcuc/ thread needs to wake up many offload threads, for instance in a large box with dozens CPUs, this can cause an interface that can be higher than the 150 microseconds we guarantee in the -rt products.

As we can enable the rcu no callbacks via kernel command line, we still can have the same behavior of CONFIG_RCU_NOCB_CPU_ALL=y by using the rcu_nocbs=<allcpus>. So it is safe to turn it off. In fact, this gives us more flexibility than have all CPUs rcu_nocbs by default
Comment 1 Clark Williams 2016-06-23 18:58:03 UTC 
This change was made in:

kernel-rt-3.10.0-439.rt56.327.el7
Comment 7 errata-xmlrpc 2016-11-03 19:52:58 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.

https://rhn.redhat.com/errata/RHSA-2016-2584.html