#linux ftrace 简介


[linuxcon2020 ftrace]: https://events.static.linuxfound.org/slides/2010/linuxcon_japan/linuxcon_jp2010_rostedt.pdf  "ftrace"


# ftrace背景

ftrace 是 Function Trace的意思, 最开始主要用于记录内核函数的运行轨迹
随着功能的逐渐增加，演变成一个用于trace的framework.

## static tracepoints
static tracepoints within the kernel (event tracing)
    scheduling
    interrupts
    file systems
    ...

## dynamic kernel funciton tracing
    trace all functions within the kernel
    pick and choose what function to trace (less functions, less overhead)
    call graph
    stack usage
    
## Latency Tracer
	how long interrupts are disabled 
	how long preemption is disabled
	how long interrupts and/or preemption is disable

## Wakeup latency
	how long it takes a process to run after it is woken

ftrace的帮助文档在 Documentation/trace
ftrace的代码在 kernel/trace
ftrace的作者在LinuxCon有一篇关于ftrace的slides ![Ftrace Linux Kernel Tracing][linuxcon2020 ftrace]


# ftrace的配置和使用

ftrace通过debugfs文件系统向用户空间提供访问接口,因此要确保挂载上debugfs
/sys/kernel/debug/tracing目录下提供了各种跟踪器(tracer)和event事件

## 通用配置

### trace
  The static contents of the buffer, 保存trace到的信息
  `echo > trace` 可以清空buffer

### trace_pipe
  和trace类似，可查看trace到的内容
  但是输出trace的同时，会将输出的信息从buffer中删除

### available_tracers
  列出支持的tracer (跟踪器)
  
----
| TRACER         | description                            |
| -              | -                                      |
| function       | 跟踪内核函数执行情况                   |
| function_graph | 可以显示函数调用关系图                 |
| wakeup         | 跟踪进程唤醒信息                       |
| wakeup_rt      |                                        |
| wakeup_dl      |                                        |
| nop            | 不跟踪任何信息,将nop写入current_tracer |
|                | 可以清空之前收集到的tracer信息         |
| mmiotrace      |                                        |
| blk            |                                        |
----

### current_tracer
  设置和显示当前正在使用的tracer, 默认为nop, 表示不做任何跟踪操作

### trace_clock
  当前trace的timestamp的时钟,默认使用local时钟

| trace_clock | description                                      |
| -           | -                                                |
| local       | Per cpu clock but may not be synced across CPUs  |
|             | 可能无法在不同cpu间同步                          |
| global      | Synced across CPUs but slows tracing down.       |
|             | 支持在不同cpu间同步                              |
| counter     | Not a clock, but just an increment               |
|             | 跨CPU计数器,对于分析不同cpu间的event顺序比较有效 |
| uptime      | Jiffy counter from time of boot                  |
| perf        | Same clock that perf events use                  |
| x86-tsc     | TSC cycle counter                                |

### tracing_on
  控制tracer打开或停止
  > echo 0 > tracing_on : quick way to disable tracing
  > echo 1 > tracing_on : quick way to re-enable tracing