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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>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.