Re: [patch] Performance Counters for Linux, v4

[Andi Kleen]

Paul Mackerras <paulus@xxxxxxxxx> writes:
>
> The perf counter subsystem will, in Ingo's design, naturally try to
> schedule as many counters and groups on as it can.  Given a list of
> counters/groups, it could start with the first and keep on trying to
> add counters or groups while it can, essentially trying all possible
> combinations until it either fills up all the hardware counters or
> exhausts the possible combinations.  If it moves all the
> counters/groups that do fit on up to the head of the list, and then
> rotates them to the back of the list when the timeslice expires, that
> would probably be OK.  In fact the computation about what set of
> counters/groups to put on should be done when adding/removing a
> counter/group and when the timeslice expires, rather than at context
> switch time.  (I'm talking about the list of part-time counters/groups
> here, of course.)

One issue is that PMU counts can cover more than one CPU. One example
for this are the Uncore events on Nehalem (which cover a whole socket)
or when you are in AnyThreads monitoring mode (then you get events
from both SMT siblings in a core)

With that you would need to examine other CPU's state at context switch
time. Probably not a good idea for scalability.

-Andi

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