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    SubjectRe: [PATCH 2/2] perf_counter: optimize context switch between identical inherited contexts
    Peter Zijlstra writes:

    > On Fri, 2009-05-22 at 14:27 +1000, Paul Mackerras wrote:
    > > Since we don't have individual fds for the counters in a cloned
    > > context, the only thing that can make two clones of a given parent
    > > different after they have been cloned is enabling or disabling all
    > > counters with prctl. To account for this, we keep a count of the
    > > number of enabled counters in each context. Two contexts must have
    > > the same number of enabled counters to be considered equivalent.
    >
    > Curious point that.. so prctl() can disable counters it doesn't own.
    >
    > Shouldn't we instead fix that?

    Well, prctl enables/disables the counters that are counting on the
    current process, regardless of who or what created them. I always
    thought that was a little strange; maybe it is useful to be able to
    disable all the counters that any other process might have put on to
    you, but I can't think of a scenario where you'd really need to do
    that, particularly since the disable is a one-shot operation, and
    doesn't prevent new (enabled) counters being attached to you.

    On the other hand, what does "all the counters I own" mean? Does it
    mean all the ones that I have fds open for? Or does it mean all the
    ones that I created? Either way we don't have a good way to enumerate
    them.

    Paul.


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