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    SubjectRe: [Lse-tech] Bug in sys_sched_yield
    Walt Drummond wrote:
    > george anzinger writes:
    > > Uh... I do know about this map, but I wonder if it is at all needed.
    > > What is the real difference between a logical cpu and the physical one.
    > > Or is this only interesting if the machine is not Smp, i.e. all the cpus
    > > are not the same? It just seems to me that introducing an additional
    > > mapping just slows things down and, if all the cpus are the same, does
    > > not really do anything. Of course, I am assuming that ALL usage would
    > > be to the logical :)
    > Right. That is not always the case. IA32 is somewhat special. ;) The
    > logical mapping allows you to, among other things, easily enumerate
    > over the set of active processors without having to check if a
    > processor exists at the current processor address.
    > The difference is apparent when the physical CPU ID is, say, an
    > address on a processor bus, or worse, an address on a set of processor
    > busses. Take a look at the IA-64's smp.h. The IA64 physical
    > processor ID is a 64-bit structure that has to 8-bit ID's; an EID for
    > what amounts to a "processor bus" ID and an ID that corresponds to a
    > specific processor on a processor bus. Together, they're a system
    > global ID for a specific processor. But there is no guarantee that
    > the set of global ID's will be contiguous.
    > It's possible to have disjoint (non-contiguous) physical processor
    > ID's if a processor bus is not completely populated, or there is an
    > empty processor slot or odd processor numbering in firmware, or
    > whatever.
    All that is cool. Still, most places we don't really address the
    processor, so the logical cpu number is all we need. Places like
    sched_yield, for example, should be using this, not the actual number,
    which IMO should only be used when, for some reason, we NEED the hard
    address of the cpu. I don't think this ever has to leak out to the
    common kernel code, or am i missing something here.

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