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.