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Date: Sun, 16 Aug 2015 13:58:59 +0200
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Nicholas Mc Guire <der.herr@hofr.at>, Clark Williams <williams@redhat.com>,
        RT <linux-rt-users@vger.kernel.org>,
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Subject: Re: [RT] oops in 4.1.3-rt3
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* Thomas Gleixner | 2015-08-04 14:05:10 [+0200]:

>On Tue, 4 Aug 2015, Nicholas Mc Guire wrote:
>> <snip>
>>     3 root      -2   0       0      0      0 R  60.3  0.0   5756:16 ksoftirqd/0 
>>    23 root      -2   0       0      0      0 R  50.8  0.0   4948:08 ksoftirqd/2
>>    17 root      -2   0       0      0      0 S  50.6  0.0   4897:13 ksoftirqd/1
>>    29 root      -2   0       0      0      0 S  50.4  0.0   4953:24 ksoftirqd/3
>
>So ksoftirqd eats 50+% CPU on each core. On an idle system!?! Any
>chance that you can get a function trace snapshot out of it?

This might be a .config thing, device driver or user land behaviour.
Usually ksoftirqd should remain (almost) idle because most of BH work
remains in task-context and not in ksoftirqd. tasklets for instance
should run in ksoftirqd. So a function trace or event tracing to figure
out what is scheduling the softirq might give a pointer.
I have here a AMD box with

 3 root      -2   0       0      0      0 S   1.3  0.0 126:16.31 ksoftirqd/0                                                                                          
18 root      -2   0       0      0      0 S   1.3  0.0 136:55.85 ksoftirqd/1                                                                                          
30 root      -2   0       0      0      0 S   1.3  0.0 138:50.52 ksoftirqd/3                                                                                          
24 root      -2   0       0      0      0 S   1.0  0.0 143:54.28 ksoftirqd/2                                                                                          

with a 15 days uptime. That one percent CPU usage on a idle system looks
odd here, too.

>Thanks,
>
>	tglx

Sebastian
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