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Oracle Solaris 11.1 Dynamic Tracing Guide     Oracle Solaris 11.1 Information Library
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Document Information

Preface

1.  About DTrace

2.  D Programming Language

3.  Aggregations

4.  Actions and Subroutines

5.  Buffers and Buffering

6.  Output Formatting

7.  Speculative Tracing

Speculation Interfaces

Creating a Speculation

Using a Speculation

Committing a Speculation

Discarding a Speculation

Speculation Example

Speculation Options and Tuning

8.  dtrace(1M) Utility

9.  Scripting

10.  Options and Tunables

11.  Providers

12.  User Process Tracing

13.  Statically Defined Tracing for User Applications

14.  Security

15.  Anonymous Tracing

16.  Postmortem Tracing

17.  Performance Considerations

18.  Stability

19.  Translators

20.  Versioning

Index

Speculation Example

One potential use for speculations is to highlight a particular code path. The following example shows the entire code path under the open(2) system call only when the open fails:

Example 7-1 specopen.d: Code Flow for Failed open(2)

#!/usr/sbin/dtrace -Fs

syscall::openat:entry,
syscall::openat64:entry
{
        /*
         * The call to speculation() creates a new speculation.  If this fails,
         * dtrace(1M) will generate an error message indicating the reason for
         * the failed speculation(), but subsequent speculative tracing will be
         * silently discarded.
         */
        self->spec = speculation();
        speculate(self->spec);

        /*
         * Because this printf() follows the speculate(), it is being 
         * speculatively traced; it will only appear in the data buffer if the
         * speculation is subsequently commited.
         */
        printf("%s", stringof(copyinstr(arg1)));
}

fbt:::
/self->spec/
{
        /*
         * A speculate() with no other actions speculates the default action:
         * tracing the EPID.
         */
        speculate(self->spec);
}

syscall::openat:return,
syscall::openat64:return
/self->spec/
{
        /*
         * To balance the output with the -F option, we want to be sure that
         * every entry has a matching return.  Because we speculated the
         * open entry above, we want to also speculate the open return.
         * This is also a convenient time to trace the errno value.
         */
        speculate(self->spec);
        trace(errno);
}

syscall::openat:return,
syscall::openat64:return
/self->spec && errno != 0/
{
        /*
         * If errno is non-zero, we want to commit the speculation.
         */
        commit(self->spec);
        self->spec = 0;
}

syscall::openat:return,
syscall::openat64:return
/self->spec && errno == 0/
{
        /*
         * If errno is not set, we discard the speculation.
         */
        discard(self->spec);
        self->spec = 0;
}

Running the above script produces output similar to the following example:

# ./specopen.d
dtrace: script './specopen.d' matched 24282 probes
CPU FUNCTION                                 
  1  => open                                  /var/ld/ld.config
  1    -> open                                
  1      -> copen                             
  1        -> falloc                          
  1          -> ufalloc                       
  1            -> fd_find                     
  1              -> mutex_owned               
  1              <- mutex_owned               
  1            <- fd_find                     
  1            -> fd_reserve                  
  1              -> mutex_owned               
  1              <- mutex_owned               
  1              -> mutex_owned               
  1              <- mutex_owned               
  1            <- fd_reserve                  
  1          <- ufalloc                       
  1          -> kmem_cache_alloc              
  1            -> kmem_cache_alloc_debug      
  1              -> verify_and_copy_pattern   
  1              <- verify_and_copy_pattern   
  1              -> file_cache_constructor    
  1                -> mutex_init              
  1                <- mutex_init              
  1              <- file_cache_constructor    
  1              -> tsc_gethrtime             
  1              <- tsc_gethrtime             
  1              -> getpcstack                
  1              <- getpcstack                
  1              -> kmem_log_enter            
  1              <- kmem_log_enter            
  1            <- kmem_cache_alloc_debug      
  1          <- kmem_cache_alloc              
  1          -> crhold                        
  1          <- crhold                        
  1        <- falloc                          
  1        -> vn_openat                       
  1          -> lookupnameat                  
  1            -> copyinstr                   
  1            <- copyinstr                   
  1            -> lookuppnat                  
  1              -> lookuppnvp                
  1                -> pn_fixslash             
  1                <- pn_fixslash             
  1                -> pn_getcomponent         
  1                <- pn_getcomponent         
  1                -> ufs_lookup              
  1                  -> dnlc_lookup           
  1                    -> bcmp                
  1                    <- bcmp                
  1                  <- dnlc_lookup           
  1                  -> ufs_iaccess           
  1                    -> crgetuid            
  1                    <- crgetuid            
  1                    -> groupmember         
  1                      -> supgroupmember    
  1                      <- supgroupmember
  1                    <- groupmember         
  1                  <- ufs_iaccess           
  1                <- ufs_lookup              
  1                -> vn_rele                 
  1                <- vn_rele                 
  1                -> pn_getcomponent         
  1                <- pn_getcomponent         
  1                -> ufs_lookup              
  1                  -> dnlc_lookup           
  1                    -> bcmp                
  1                    <- bcmp                
  1                  <- dnlc_lookup           
  1                  -> ufs_iaccess           
  1                    -> crgetuid            
  1                    <- crgetuid            
  1                  <- ufs_iaccess           
  1                <- ufs_lookup              
  1                -> vn_rele                 
  1                <- vn_rele                 
  1                -> pn_getcomponent         
  1                <- pn_getcomponent         
  1                -> ufs_lookup              
  1                  -> dnlc_lookup           
  1                    -> bcmp                
  1                    <- bcmp                
  1                  <- dnlc_lookup           
  1                  -> ufs_iaccess           
  1                    -> crgetuid            
  1                    <- crgetuid            
  1                  <- ufs_iaccess           
  1                  -> vn_rele               
  1                  <- vn_rele               
  1                <- ufs_lookup              
  1                -> vn_rele                 
  1                <- vn_rele                 
  1              <- lookuppnvp                
  1            <- lookuppnat                  
  1          <- lookupnameat                  
  1        <- vn_openat                       
  1        -> setf                            
  1          -> fd_reserve                    
  1            -> mutex_owned                 
  1            <- mutex_owned                 
  1            -> mutex_owned                 
  1            <- mutex_owned                 
  1          <- fd_reserve                    
  1          -> cv_broadcast                  
  1          <- cv_broadcast                  
  1        <- setf                            
  1        -> unfalloc                        
  1          -> mutex_owned                   
  1          <- mutex_owned                   
  1          -> crfree                        
  1          <- crfree                        
  1          -> kmem_cache_free               
  1            -> kmem_cache_free_debug       
  1              -> kmem_log_enter            
  1              <- kmem_log_enter            
  1              -> tsc_gethrtime             
  1              <- tsc_gethrtime             
  1              -> getpcstack                
  1              <- getpcstack                
  1              -> kmem_log_enter            
  1              <- kmem_log_enter
  1              -> file_cache_destructor     
  1                -> mutex_destroy           
  1                <- mutex_destroy           
  1              <- file_cache_destructor     
  1              -> copy_pattern              
  1              <- copy_pattern              
  1            <- kmem_cache_free_debug       
  1          <- kmem_cache_free               
  1        <- unfalloc                        
  1        -> set_errno                       
  1        <- set_errno                       
  1      <- copen                             
  1    <- open                                
  1  <= open                                          2