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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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