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    SubjectRe: /sys/devices/system/cpu/*: Present cpus or Possible cpus
    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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