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Sync option comment with kernel config.

DTrace is a Dynamic Tracing framework developed by Sun and ported to NetBSD. It enables extensive instrumentation of the kernel and user space. See the [DTrace Community Page](http://dtrace.org) for more information. Also see [DTrace Introduction](http://dtrace.org/guide/preface.html). 

# Current status

## Supported platforms

DTrace is a work-in-progress effort and it is for x86 systems and some arm boards.

* i386 and amd64
* evbarm
    * BEAGLEBONE and SHEEVAPLUG

## Supported providers

* SDT: Statically Defined Tracing
* FBT: Function Boundary Tracing

You can currently run a hello world DScript. 

## TODO for netbsd-7

* Rename provider modules to `dtrace_*.kmod`: `dtrace_fbt.kmod`, &c.
* Measure effect of `options KDTRACE_HOOKS` on system performance.
* Determine whether the profile module works and list it here.
* Put a dtrace target in /dev/MAKEDEV.
* Integrate [[riz|users/riz]]'s syscall provider patch.

## TODO for netbsd-6

Need to identify changes to pull up to netbsd-6 and pull them up.
Candidates:

* Profile provider.

# How to use

##  Building DTrace 

You need the following options in your kernel: 
    
    options         INSECURE
    options         KDTRACE_HOOKS   # kernel DTrace hooks
    options         MODULAR
    

You also need to build distribution with the options MKDTRACE=yes.

##  Running hello world 

Load the solaris and dtrace modules, and the SDT (Statically Defined Tracing) and FBT (Function Boundary Tracing) modules: 
    
    modload solaris
    modload dtrace
    modload sdt
    modload fbt
    

Make the dtrace device node: 
    
    mkdir /dev/dtrace
    mknod /dev/dtrace/dtrace c dtrace 0
    

List the dtrace probes 
    
    dtrace -l
    
       ID   PROVIDER            MODULE                          FUNCTION NAME
        1     dtrace                                                     BEGIN
        2     dtrace                                                     END
        3     dtrace                                                     ERROR
        4        fbt            netbsd             AcpiAcquireGlobalLock entry
        5        fbt            netbsd             AcpiAcquireGlobalLock return
        6        fbt            netbsd             AcpiAllocateRootTable entry
        7        fbt            netbsd                    AcpiAttachData entry
        . 
        .
    29129        fbt           solaris                   zfs_vop_getattr entry 
    29130        fbt           solaris                   zfs_vop_getattr return
    29131       proc                                                     create
    29132       proc                                                     exec 
        .
        .
    29140       proc                                                     lwp_start
    29141       proc                                                     lwp_exit
    

  


Put the following into the file hello.d:
    
    BEGIN
    {
        trace("Hello world");
        exit(0);
    }
    

Run the hello world script: 
    
    dtrace -s hello.d
    
    dtrace: script './hello.d' matched 1 probe
    CPU     ID                    FUNCTION:NAME
      0      1                           :BEGIN   Hello world
    

A more complex example that traces the execution of a sleep operation
in the kernel. Put it in sleep.d:
    
    #pragma D option flowindent
    
    fbt::syscall:entry
    /execname == "sleep" && guard++ == 0/
    {
            self->traceme = 1;
            printf("fd: %d", arg0);
    }
    
    fbt::syscall:entry /self->traceme/ {}
    
    fbt::syscall:return
    /self->traceme/
    {
            self->traceme = 0;
            exit(0);
    }
    

Start the script running (dtrace -s sleep.d) and then execute a "sleep 2" in another shell.