debug/gprof.rst

.. _gprof:

Gprof
#####
This describes to do profiling of user Trusted Applications with ``gprof``.

The configuration option ``CFG_TA_GPROF_SUPPORT=y`` enables OP-TEE to collect
profiling information from Trusted Applications running in user mode and
compiled with ``-pg``. Once collected, the profiling data are formatted in the
``gmon.out`` format and sent to ``tee-supplicant`` via RPC, so they can be saved
to disk and later processed and displayed by the standard ``gprof`` tool.

Usage
*****

    - Build OP-TEE OS with ``CFG_TA_GPROF_SUPPORT=y``. You may also set
      ``CFG_ULIBS_MCOUNT=y`` to instrument the user TA libraries contained in
      ``optee_os`` (such as ``libutee`` and ``libutils``).

    - Build user TAs with ``-pg``, for instance enable: ``CFG_TA_MCOUNT=y`` to
      instrument whole user TA. Note that instrumented TAs have a larger
      ``.bss`` section. The memory overhead is 1.36 times the ``.text`` size for
      32-bit TAs, and 1.77 times for 64-bit ones (refer to the TA linker script
      for details: ``ta/arch/arm/ta.ld.S``).

    - Run the application normally. When the last session exits,
      ``tee-supplicant`` will write profiling data to
      ``/tmp/gmon-<ta_uuid>.out``. If the file already exists, a number is
      appended, such as: ``gmon-<ta_uuid>.1.out``.

    - Run gprof on the TA ELF file and profiling output: ``gprof <ta_uuid>.elf
      gmon-<ta_uuid>.out``

Implementation
**************
Part of the profiling is implemented in libutee. Another part is done in the TEE
core by a pseudo-TA (``core/arch/arm/sta/gprof.c``). Two types of data are
collected:

    1. Call graph information
        - When TA source files are compiled with the -pg switch, the compiler
          generates extra code into each function prologue to call the
          instrumentation entry point (``__gnu_mcount_nc`` or ``_mcount``
          depending on the architecture). Each time an instrumented function is
          called, libutee records a pair of program counters (one is the caller
          and the other one is the callee) as well as the number of times this
          specific arc of the call graph has been invoked.

    2. PC distribution over time
        - When an instrumented TA starts, libutee calls the pseudo-TA to start
          PC sampling for the current session. Sampling data are written into
          the user-space buffer directly by the TEE core.

        - Whenever the TA execution is interrupted, the TEE core records the
          current program counter value and builds a histogram of program
          locations (i.e., relative amount of time spent for each value of the
          PC). This is later used by the gprof tool to derive the time spent in
          each function. The sampling rate, which is assumed to be roughly
          constant, is computed by keeping track of the time spent executing
          user TA code and dividing the number of interrupts by the total time.

        - The profiling buffer into which call graph and sampling data are
          recorded is allocated in the TA's ``.bss`` section. Some space is
          reserved by the linker script, only when the TA is instrumented.