There is no good reason why we cannot synthesize "cycle" events from
Intel PT just as we can synthesize "instruction" events, in particular
when CYC packets are available. This enables using PT to getting much
more accurate cycle profiles than regular sampling (record -e cycles)
when the work last for very short periods (<10 ms). Thus, add support
for this, based off of the existing IPC calculation framework. The new
option to --itrace is "y" (for cYcles), as c was taken for calls. Cycle
and instruction events can be synthesized together, and are by default.
The only real caveat is that CYC packets are only emitted whenever some
other packet is, which in practice is when a branch instruction is
encountered (and not even all branches). Thus, even at no subsampling
(e.g. --itrace=y0ns), it is impossible to get more accuracy than a
single basic block, and all cycles spent executing that block will get
attributed to the branch instruction that ends the packet. Thus, one
cannot know whether the cycles came from e.g. a specific load, a
mispredicted branch, or something else. When subsampling (which is the
default), the cycle events will get smeared out even more, but will
still be generally useful to attribute cycle counts to functions.
Reviewed-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Steinar H. Gunderson <sesse@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220322082452.1429091-1-sesse@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
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