Messages in this thread | | | Date | Wed, 2 May 2007 11:37:57 -0500 | From | Nathan Lynch <> | Subject | Re: /sys/devices/system/cpu/*: Present cpus or Possible cpus |
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Hi Gautham-
Gautham R Shenoy wrote: > > Looking at the topology_init() code, I observe that the meaning of > the cpuX/ directory entries in /sys/devices/system/cpu/ might be > different for different architectures. > > Looks like, in case of i386, ia64, m32, mips etc, the cpuX directory entries > represent the "present cpus". > > However, in case of powerpc, s390 etc, the cpuX entries represent the > "possible cpus". > > Wondering if there is any particular reason for this discrepancy.
I believe that the powerpc behavior was established before cpu_present_map was introduced.
> I am not entirely surely if it's due cpu hotplug because > both i386 and powerpc support it!
powerpc also supports processor add and remove (as opposed to online/offline); i386 does not AFAIK. I think this may be a reason for the difference.
> When I do a > "echo 1 > /sys/devices/system/cpu/cpuX/online" on a power box as root, > I might get "-bash: echo: write error: Invalid argument" > because cpuX might not be present! > > In case of lpar, cpu_present_map need not necessarily be equal to > cpu_possible_map, so the above error is observable.
Working as intended. You have to add a cpu to the partition before you can online it.
> Is this discrepency intentional ? > Or is it due to the fact that in most cases, > cpu_present_map == cpu_possible_map, so lets not bother about it :-?
I think it's the inevitable result when architectures are free to invent their own versions of the same sysfs interface. But is it really causing a problem in this case?
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