Bug 1430074

Summary: Backport "net: dev: always take qdisc's busylock in __dev_xmit_skb" upstream RT patch
Product: Red Hat Enterprise Linux 7 Reporter: Arnaldo Carvalho de Melo <acme>
Component: kernel-rtAssignee: Arnaldo Carvalho de Melo <acme>
kernel-rt sub component: Networking QA Contact: Jiri Kastner <jkastner>
Status: CLOSED ERRATA Docs Contact:
Severity: medium    
Priority: high CC: bhu, jkacur, lgoncalv, ovasik, williams
Version: 7.4   
Target Milestone: rc   
Target Release: ---   
Hardware: Unspecified   
OS: Unspecified   
Whiteboard:
Fixed In Version: Doc Type: No Doc Update
Doc Text:
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Clone Of: Environment:
Last Closed: 2017-08-01 19:02:59 UTC Type: Bug
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: --- Target Upstream Version:
Embargoed:
Bug Depends On:    
Bug Blocks: 1353018, 1410158    

Description Arnaldo Carvalho de Melo 2017-03-07 19:44:41 UTC 
Description of problem:

The 4.8.15-rt10 (real-time kernel patchset) contains a patch called

 net: dev: always take qdisc's busylock in __dev_xmit_skb()

Its a one-liner that, its description says:

<quote>
The root-lock is dropped before dev_hard_start_xmit() is invoked and after
setting the __QDISC___STATE_RUNNING bit. If this task is now pushed away
by a task with a higher priority then the task with the higher priority
won't be able to submit packets to the NIC directly instead they will be
enqueued into the Qdisc. The NIC will remain idle until the task(s) with
higher priority leave the CPU and the task with lower priority gets back
and finishes the job.

If we take always the busylock we ensure that the RT task can boost the
low-prio task and submit the packet.
</>

So its only activated for CONFIG_PREEMPT_RT_FULL and should let the RT core algorithms to take a better decision in this case.
Comment 5 errata-xmlrpc 2017-08-01 19:02:59 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://access.redhat.com/errata/RHSA-2017:2077
Comment 6 errata-xmlrpc 2017-08-02 00:25:36 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://access.redhat.com/errata/RHSA-2017:2077