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    SubjectRe: High load average on idle machine running 2.6.32
    [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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