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    SubjectRe: High CPU load when machine is idle (related to PROBLEM: Unusually high load average when idle in 2.6.35, 2.6.35.1 and later)
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    On Thu, 2010-10-14 at 16:58 +0200, Damien Wyart wrote:
    > Hello,
    >
    > > With 2.6.36-rc6, I'm seeing a load of around 0.60 when the machine is
    > > completely idle. This is similar to what someone reported for the latest
    > > 2.6.35.x stables. This is on a core i7 machine, but I've no time to
    > > bisect or test earlier versions right now, but I guess this is easy to
    > > reproduce on the same plateform.
    >
    > After further investigation and cross-checking with the thread "PROBLEM:
    > Unusually high load average when idle in 2.6.35, 2.6.35.1 and later",
    > I came to the following results:
    >
    > - the commit 74f5187ac873042f502227701ed1727e7c5fbfa9 isolated by Tim
    > seems to be the culprit;
    > - reverting it solves the problem with 2.6.36-rc7 in NOHZ mode: the load
    > when idle goes down to 0.00 (which it never does with the patch
    > applied)
    > - using nohz=no with the commit reverted still gives correct behaviour
    > (tested just in case)
    >
    > - vanilla 2.6.36-rc7 with this commit applied has the problem (load is
    > around 0.60 when machine idle, sometimes less after several hours
    > of uptime, but never 0.00), and rebooting with nohz=no makes the
    > problem disappear: load goes down to 0.00 quickly after boot process
    > has finished.
    >
    > I hope this answers the questions raised in the Tim's thread.
    >
    > Could someone with knowledge of the commit take a look at the problem?
    > It would be a bit annoying to have this problem in 2.6.36, since Tim's
    > initial report dates back to 2 weeks ago...
    >
    > I can help for further testing if needed.

    Sorry I haven't been as responsive to this issue as I would have liked.
    I've been rather busy on other work.

    My biggest testing concern is that in reality, load on a normal desktop
    machine (i.e. not some stripped down machine disconnected from network
    or any other input running nothing but busybox) should not be 0.00.
    Maybe load on a server doing absolutely nothing could be 0.00, but
    there's usually something going on that should bump it up to a few
    hundredths. Watch top, and if you see at least 1% constant cpu usage
    there, your load average should be at least 0.01. That said, there does
    seem to be a bug somewhere as a load of 0.60 on an idle machine seems
    high.

    -- Chase



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