[DTrace-devel] BPF_F_CURRENT_CPU

[Kris Van Hees]

Cc: to the dtrace-devel list because it is an important discussion.

On Tue, May 12, 2020 at 06:53:31PM -0700, Eugene Loh wrote:
> In our BPF function prologue, we call a BPF helper function to get the
> current CPU, then convert to 32-bit, then store to the stack.  In the
> epilogue, we retrieve that value to pass to bpf_perf_event_output().
> 
> Wouldn't it be simpler just to pass BPF_F_CURRENT_CPU to
> bpf_perf_event_output()?  Or will we want the current CPU on the stack
> anyhow for other reasons (e.g., built-in variable)?

Yes, you are right.  When I wrote that code originally, I had more uses for
the current cpu index (e.g. obtaining scratch memory to construct the output
record) and indeed inlined resolving of built-in variables.  And since I had
it anyway I never even looked to see if there was a mode for
bpf_perf_event_output() to output to the buffer for the current cpu without
needing to explicitly specify it.

Since you found this convenient way to not needing to know the cpu index and
since we actually do not have any other need for it anymore (the builtin
variable need to be handled different anyway because it provides a struct of
data rather than just the cpu index), it would be a good idea to rework that
code to no longer retrieve and store the cpu index.  That also frees up a
little bit of space in the dt_bpf_context.

Do you want to tackle that reworking?

	Kris