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Bug#674153: [3.2.16 -> 3.2.17 regression] High reported CPU load when idle

On Thu, 2012-07-19 at 11:12 +0200, Lesław Kopeć wrote:
> On 07/18/2012 01:25 AM, Jonathan Nieder wrote:
> > Anders Boström wrote:
> > 
> >> Starting with 3.2.17-1, the CPU load accounting is broken when the
> >> computer is idle. The CPU load is reported as >0.50 when
> >> idle. 3.2.16-1 don't suffer from this problem.
> >>
> >> Suspected patch is the upstream patch
> >> "sched: Fix nohz load accounting -- again!"
> >> commit 5e2d50da11f0e6ec3ce8fe658d7c83b0b4346c68 to 3.2 and
> >> originating from c308b56b5398779cd3da0f62ab26b0453494c3d4 .
> > 
> > Please test the attached patch against a 3.2.y kernel, for example
> > following the instructions below:
> Good news everyone. I have tested kernel 3.2.21 and the attached patch
> (based on 5167e8d I presume) seems to be fixing all the load average
> oddities. I've compiled following kernels:
> * 3.2.21-hz		(CONFIG_NO_HZ=n)
> * 3.2.21-no-hz		(CONFIG_NO_HZ=y)
> * 3.2.21-no-hz-5167e8d	(CONFIG_NO_HZ=y) + attached patch
> The load reported by 3.2.21-hz and 3.2.21-no-hz-5167e8d is exactly the
> same under different CPU usage. Without the patch the tickless kernel
> tends to show lower load values than what you would expect.
> I can't say much for the case when load is too high on an idle machine,
> because I haven't been able to reproduce the problem in the first place.
> To summarize: the bug is present in unpatched kernel and fixed by
> applying the attached patch. No nasty side effects noticed.

This is in the review queue for Linux 3.2.24.  I'm hesistant to apply it
until it's been through the stable review process (probably early next
week).  But, if there's no objection to it there, it will end up in
Debian pretty soon.


Ben Hutchings
DNRC Motto:  I can please only one person per day.
Today is not your day.  Tomorrow isn't looking good either.

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