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    SubjectRe: Bogus load average and cpu times on x86_64 SMP kernels
    Brian Gerst wrote:
    > I've been seeing bogus values from /proc/loadavg on an x86-64 SMP kernel
    > (but not UP).
    > $ cat /proc/loadavg
    > -1012098.26 922203.26 -982431.60 1/112 2688
    > This is in the current git tree. I'm also seeing strange values in
    > /proc/stat:
    > cpu 2489 40 920 60530 9398 171 288 1844674407350
    > cpu0 2509 60 940 60550 9418 191 308 0
    > The first line is the sum of all cpus (I only have one), so it's picking
    > up up bad data from the non-present cpus. The last value, stolen time,
    > is completely bogus since that value is only ever used on s390.
    > It looks to me like there is some problem with how the per-cpu
    > structures are being initialized, or are getting corrupted. I have not
    > been able to test i386 SMP yet to see if the problem is x86_64 specific.

    I found the culprit: CPU hotplug. The problem is that
    prefill_possible_map() is called after setup_per_cpu_areas(). This
    leaves the per-cpu data sections for the future cpus uninitialized
    (still pointing to the original per-cpu data, which is initmem). Since
    the cpus exists in cpu_possible_map, for_each_cpu will iterate over them
    even though the per-cpu data is invalid.

    Brian Gerst
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      Last update: 2005-10-05 20:02    [W:0.022 / U:0.336 seconds]
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