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SubjectPer-CPU data as a structure
Date
Hello,

At the moment, data specific to a CPU is stored in different, fixed-
size separate arrays by means of the "percpu framework". I'm working
on some changes to modify the way some CPUs are represented, and I'm
wondering what's the rationale behind such a representation.

At first sight, it'd seem more reasonable to have a structure holding
all the information that is CPU-specific (as is done with any object
represented within the system). After searching the mail archives, I
see that similar changes were proposed before, but those threads did
not seem to get any reply (so I'm assuming that the changes were not
desired).

Similarly, and if I understood it correctly, the PDA (Per-processor
Data Area) also aims to do the above, but at the moment it only
contains some fields and is not defined in all platforms. There are
still a lot of usages of the percpu functionality (such as, e.g., in
kernel/sched.c).

Part of my changes introduce a new structure that is able to
represent any kind of CPU (and which each platform can extend to add
new information to it). It is supposed to supersede the per-cpu
definitions. I bet this could also be redone by using percpu in some
way... The thing is I am willing to share my work when I've finished
it (it is still very much work-in-progress), but first I'm interested
to know if adding this new structure is a crazy idea (meaning I
should stick to percpu wherever possible) or something that can be
accepted later on.

Summarizing, my questions are:
- Why is the code currently using multiple separate arrays (percpu)
to hold CPU information instead of a structure?
- Could this structure-based approach (instead of all these separate
arrays) be considered for inclusion into the system?

As far as I can tell, the advantage of percpu is that you can define
new "fields" anywhere in the code and independently from the rest of
the system. Also, I seem to understand that there are performance
advantages related to this. But on the other hand, percpu seems like
an unnatural approach to "reimplement" regular structures.

Thank you very much.

--
Julio M. Merino Vidal <jmerino@ac.upc.edu>


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