Messages in this thread | | | Date | Fri, 07 Oct 2005 11:27:52 -0400 | From | Brian Gerst <> | Subject | Re: [PATCH] Fix hotplug cpu on x86_64 |
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Protasevich, Natalie wrote: >>Brian Gerst wrote: >> >>>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. >> > > I had to do the same in i386, but initially I was trying to avoid the > whole situation - allocating per_cpu data for all possible processors. > It seemed wasteful that on the system with NR_CPU=256 or 512 and brought > up as 4x everything per_cpu is (pre)allocated for all, although it's > sure convenient. I though at the time it would be great if > alloc_percpu() mechanism was able to dynamically re-create all the > per_cpu's for new processors, that way cpu_possible_map woun't probably > even be needed. Or is it too much trouble for too little gain... > > Thanks, > --Natalie >
It certainly is possible. In the hotplug cpu case, don't put the .data.percpu section in __initmem. It will then be preserved for any cpus that come online after boot.
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