Re: High load average on idle machine running 2.6.32

[Andrea Suisani <sickpig@opinioni.net>]

[cc:ed Peter Zijlstra]

James Pearson wrote:
> James Pearson wrote:
> 
>>> I've booted a 64 bit 2.6.32 kernel on dual processor, quad core Xeon 
>>> E5440 machine. The load average when the machine is idle varies 
>>> between 2 and 3.
>>>
>>> When using a 2.6.31 kernel on the same machine, the load average when 
>>> idle is nearly 0
>>>
>>> The kernel doesn't use modules - all that is needed is compiled in. 
>>> The machine uses NFS-root
>>>
>>> Strangely, when I run 'iftop' (from 
>>> http://www.ex-parrot.com/pdw/iftop/) using the 2.6.32 kernel, the 
>>> load average drops to below 0.5  - stop running iftop, and the load 
>>> average climbs again ...
>>>
>>> Any idea what might be causing this?
>>
>>
>> It looks like whatever is causing this happened between 2.6.31-git7 
>> and 2.6.31-git8 - unfortunately I don't know how to find out what 
>> change caused this ...
>>
>> Also, if I 'hot-unplug' CPUs 1 to 7, the load average drops to 0 - 
>> when I re-enable theses CPUs, the load average climbs.
>>
>> I guess this is a problem with my particular config - or maybe because 
>> I'm using NFS-root (the root file system is readonly), or using a 
>> non-module kernel?
> 
> I gave 'git bisect' a go - which appears to suggest that my problem 
> started at:
> 
> % git bisect bad
> d7c33c4930f569caf6b2ece597432853c4151a45 is first bad commit
> commit d7c33c4930f569caf6b2ece597432853c4151a45
> Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Date:   Fri Sep 11 12:45:38 2009 +0200
> 
>     sched: Fix task affinity for select_task_rq_fair
> 
>     While merging select_task_rq_fair() and sched_balance_self() I made
>     a mistake that leads to testing the wrong task affinty.
> 
>     Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
>     LKML-Reference: <new-submission>
>     Signed-off-by: Ingo Molnar <mingo@elte.hu>
> 
> :040000 040000 3d7aa3e193c7faf9c7ebbb1443c6f63269d86d04 
> 9cfb647eb5d80f156fd8a495da68f765c3fdd772 M      kernel
> 
> 
> However, while running the bisects, it became harder to decide what was 
> a 'bad' and a 'good' idle load average - for example the kernel with the 
> above patch gave an idle load average of about 1.5 - which is not as 
> high as the idle load average seen with a 2.6.32 kernel and the kernel 
> without this patch gave an idle load average of about 0.7 - which is not 
> as low as the idle load average with a 2.6.31 kernel ...
> 
> So I guess, it is not just one patch that has caused the issue I'm 
> seeing, which I guess is to be expected as the above patch was part of 
> the 'scheduler updates for v2.6.32' patch set 
> <http://marc.info/?l=linux-kernel&m=125322428306777&w=2>
> 
> I guess as no one else has reported this issue - it must be something to 
> do with my set up - could using NFS-root affect how the load average is 
> calculated?
> 
> Or, do I have something strange or missing in my kernel config that 
> could cause this issue?
> 
> Thanks
> 
> James Pearson
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