Re: [PATCH 0/2] ftrace: updates to tip

[Ingo Molnar <mingo@elte.hu>]

* Ingo Molnar <mingo@elte.hu> wrote:

> > I was afraid you would say that :-)
> > 
> > Yes, we could add something to do this. It would take some thought on 
> > a good api, and implementation. This is not an easy task by any means. 
> > We need a way to map between a function call and a tracer, where a 
> > function call can call more than one tracer.
> 
> Note that some other things could be consolidated under per function 
> metadata: for example set_graph_function - which really is a per 
> function attribute too. Plus a lot of new things would be enabled i 
> think.

a few other things that could be per-function attributes:

 - Using function trace 'limits': trace a function 50 times and dont trace 
   it afterwards. Each invocation of that function decreases the
   remaining-count by one. For example:

     echo 'btrfs_join_transaction:#2' >> set_ftrace_filter

   Would specify that we generate two trace entries of
   btrfs_join_transaction(), then stop tracing this function.

 - Using function-triggered tracing: a function could be specified (via a 
   filter format extension) to act as a 'start tracing' trigger. Another 
   extension would be 'stop tracing' trigger.

   For example:

      echo 'btrfs_join_transaction:+' >> set_ftrace_filter
      echo 'btrfs_commit_transaction:-' >> set_ftrace_filter

   The '+' is a start-tracing trigger condition, the '-' is a stop-tracing 
   trigger condition. All function calls between btrfs_join_transaction() 
   and btrfs_commit_transaction() would be traced.

The two could be combined - to generate the trace of a single btrfs 
transaction, one could do:

      echo 0 > tracing_enabled
      echo 'btrfs_join_transaction:+#1' >> set_ftrace_filter
      echo 'btrfs_commit_transaction:-#1' >> set_ftrace_filter
      echo 1 > tracing_enabled

Other extensions are possible too:

 - Trace length triggers. For example one could do:

      echo 'btrfs_join_transaction:+*#10' >> set_ftrace_filter

   To trace 10 function calls [allowed by current filter settings] after
   the first btrfs_join_transaction() call - and stop tracing after those 
   10 trace entries.

This would allow the creation of "surgical" one-time traces - of events 
and functions one is specifically interested in.

	Ingo