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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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