Task Trace Internals

Overview

The Task Trace is constructed by the following functions.

NuttX kernel events collection

The kernel events are collected by sched_note_*() API calls embedded in NuttX kernel.

For task switch events

sched_note_start()

sched_note_stop()

sched_note_suspend()

sched_note_resume()

For system call events

sched_note_syscall_enter()

sched_note_syscall_leave()

For interrupt event

sched_note_irqhandler()

Filter logic (`nuttx/sched/sched_note.c`)

The sched_note_*() APIs are implemented here.
Filter the notes and pass them to noteram driver by sched_note_add() API.

Noteram device driver (`nuttx/drivers/note/noteram_driver.c`)

Accumurate incoming note records into the buffer.
Read the note records from the buffer by user requests.
The notes are recorded in the binary format of struct note_*_s.
The detail function is described in Note Driver Interface.

Notectl device driver (`nuttx/drivers/note/notectl_driver.c`)

/dev/notectl device driver.
Control the filter logic in sched_note.c by calling note filter APIs.
The detail function is described in Note Driver Interface.

“`trace`” Built-In Application (`apps/system/trace/trace.c`)

trace Built-In Application to control the trace function interactively.
Read binary note records from /dev/note and convert into the ftrace text format which is acceptable by “Trace Compass”.
The command syntax is described in Task Trace User Guide.

Getting the system call events

To get the system call events, two different methods are used for FLAT build and PROTECTED/KERNEL build.

FLAT build

In FLAT build, a system call is just a function call into the NuttX kernel.

To get the system call events, wrapper function option of the GNU Linker is used.

The mksyscall tool is fixed to generate the system call wrapper which call system call enter/leave hook. The wrapper supersedes the system call function call of the NuttX binary by passing --wrap linker option to the build system. The wrapper calls the system call hooks before and after calling the real system call function.

PROTECTED/KERNEL build

Different to FLAT build, in PROTECTED and KERNEL build, a system call is issued by an user space is handled as the following steps.

System call issued by an application code is handled by the system call proxy (automatically generated by mksyscall).
System call proxy issues the supervisor call instruction to enter into the kernel space.
System call handler in the kernel space calls the system call stub (automatically generated by mksyscall).
System call stub calls the API implementation in the NuttX kernel.

To get the system call events, the mksyscall tool is fixed to generate the system call wrapper which supersedes the system call function call in the system call stub.