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Date: Wed, 25 Jan 2012 13:56:22 -0500
From: "David H. Durgee" <dhdurgee@verizon.net>
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To: Mandeep Singh Baines <msb@chromium.org>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Request for assistance - excessive kworker CPU wakeups
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Mandeep Singh Baines wrote:
> On Fri, Dec 9, 2011 at 1:27 PM, David H. Durgee<dhdurgee@verizon.net>  wrote:
>> I recently purchased a Lenovo IdeaPad Z560, model 09143YU, and as I am not a
>> Windows fan I installed Linux Mint 11 Katya x64 to use instead of the
>> supplied W7.  I was encountering a known hang and had to upgrade to a later
>> kernel, so I am now using the 2.6.38-11-generic #50-Ubuntu SMP Mon Sep 12
>> 21:17:25 UTC 2011 x86_64 kernel.
>>
>> I had my first occasion to use the laptop for an extended period for the
>> first time over the Thanksgiving holiday and I found it needed to be tuned.
>>   I downloaded powertop and used it to discover where my problems were.
>>   After addressing excessive i915 interrupts due to DRI the next most
>> frequent cause of CPU wakeups is a kworker on the system.  A search lead to
>> a post by Tejun, indicating the need to trace such issues. Running the trace
>> showed that 1933 of 2748 events were of the form:
>>
>> <idle>-0     [000] 22005.355346: workqueue_queue_work: work
>> struct=ffff8800bb411188 function=do_dbs_timer workqueue=ffff88012b5d2c00
>> req_cpu=0 cpu=0
>>
>> Tejun indicated that this is a workitem used by cpufreq and likely caused by
>> something else hitting the CPU frequently.  So how do I diagnose this
>> further and isolate the cause for correction?
>>
>> If you would like a summary of this, download this spreadsheet:
>>
>> http://home.comcast.net/%7Eddurgee/Tracelog.ods
>>
>> If you would like to inspect the trace log itself:
>>
>> http://home.comcast.net/%7Eddurgee/tracelog.zip
>>
> Hi Dave,
>
> I don't know the cpufreq code that well, but it seem that this
> workqueue is schedule periodically. You
> can examine the sampling rate via /sys:
>
> $ grep "" /sys/devices/system/cpu/cpufreq/ondemand/sampling_rate*
>
> Just curious, are you running nohz:
>
> $ dmesg | grep -i nohz
>
> Regards,
> Mandeep
dhdurgee@DHD-Z560 ~/Downloads $ grep "" 
/sys/devices/system/cpu/cpufreq/ondemand/sampling_rate*
/sys/devices/system/cpu/cpufreq/ondemand/sampling_rate:10000
/sys/devices/system/cpu/cpufreq/ondemand/sampling_rate_max:4294967295
/sys/devices/system/cpu/cpufreq/ondemand/sampling_rate_min:10000
dhdurgee@DHD-Z560 ~/Downloads $ dmesg | grep -i nohz
dhdurgee@DHD-Z560 ~/Downloads $

Does this explain what I am seeing?  Does this square with the 
observations in my spreadsheet?  Does this suggest a means of reducing 
these excessive wakeups?  Is there more information I can provide to 
suggest a course of action?

Dave