Re: Bogus load average and cpu times on x86_64 SMP kernels

[Brian Gerst]

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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