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    SubjectRe: [PATCH 4/4][RFC] perf_counter: Allow sharing of output channels

    * Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

    > Provide the ability to configure a counter to send its output to
    > another (already existing) counter's output stream.
    >
    > [ compile tested only ]

    very nice!

    two comments:

    > Index: linux-2.6/include/linux/perf_counter.h
    > ===================================================================
    > --- linux-2.6.orig/include/linux/perf_counter.h
    > +++ linux-2.6/include/linux/perf_counter.h
    > @@ -216,6 +216,7 @@ struct perf_counter_attr {
    > #define PERF_COUNTER_IOC_REFRESH _IO ('$', 2)
    > #define PERF_COUNTER_IOC_RESET _IO ('$', 3)
    > #define PERF_COUNTER_IOC_PERIOD _IOW('$', 4, u64)
    > +#define PERF_COUNTER_IOC_SET_OUTPUT _IO ('$', 5)

    Time to add a new sys_perf_counter_chattr() syscall and deprecate
    the ioctls?

    > @@ -415,6 +416,9 @@ enum perf_callchain_context {
    > PERF_CONTEXT_MAX = (__u64)-4095,
    > };
    >
    > +#define PERF_FLAG_FD_NO_GROUP (1U << 0)
    > +#define PERF_FLAG_FD_OUTPUT (1U << 1)

    Why not extend the size of perf_counter_attr and add an output_fd
    parameter? Zero would mean no fd (and this is also backwards
    compatible behavior).

    FD_NO_GROUP is a bit unclean API as it aliases group_fd to two
    purposes: the real group_fd and this new output_fd. I think we
    should move output_fd to the attribute structure.

    That way one could also create a group counter which shares the
    output channel with another counter, in the same
    perf_counter_open() call.

    Ingo


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