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Date: Thu, 12 Apr 2001 10:31:34 -0700
From: george anzinger <george@mvista.com>
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To: drummond@engr.valinux.com
CC: Hubertus Franke <frankeh@us.ibm.com>, mingo@elte.hu,
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Subject: Re: [Lse-tech] Bug in sys_sched_yield
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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.

George
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