Messages in this thread | | | Date | Wed, 05 Oct 2005 14:00:36 -0400 | From | Brian Gerst <> | Subject | Re: Bogus load average and cpu times on x86_64 SMP kernels |
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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.
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