Re: [PATCH v2 0/4] ftrace: important updates

[Ingo Molnar]

* Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> On Wed, 7 Jan 2009, Pekka Paalanen wrote:
> 
> > On Wed, 7 Jan 2009 10:49:05 +0100
> > Ingo Molnar <mingo@xxxxxxx> wrote:
> > 
> > > * Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> > > 
> > > >       ring-buffer: rename debugfs file tracing_on to writing_enabled
> > > 
> > > writing_enabled is at least as confusing as tracing_on - if not more so.
> > > 
> > > The user really does not care about all the deeper machinery that happens 
> > > in ftrace - the difference between a 'light' disabling of a tracer and a 
> > > 'heavy' disabling of a tracer (which means it unregisters itself 
> > > completely in essence).
> > > 
> > > To resolve this, we should probably hide this difference altogether (as i 
> > > have suggested to do many months ago, when this first came up), by 
> > > removing tracing_enabled.
> > > 
> > > A tracer can still be fully unregistered: by simply switching the current 
> > > tracer to the 'nop' tracer. tracing_on/off remains the lightweight version 
> > > that most users are interested in anyway.
> > 
> > This sounds even better. We should not need tracing_enabled anymore, since
> > the buffer size can be changed regardless. tracing_on is more useful to
> > mmiotrace than tracing_enabled, too, since mmiotrace does not implement
> > the proper pausing behaviour on the tracing_enabled trigger.
> > 
> > But if Steven wants to keep the "pause" semantics, should there be a
> > callback dispatched to the tracer from tracing_on, just like there is
> > from tracing_enabled currently? Of course the callback semantics would
> > be different and the usual reaction would be to do nothing. It would
> > only be meaningful to tracers that update data outside the ring buffer,
> > so that they can properly pause.
> 
> The tracing_on is implemented by the ring buffer, and disables all ring 
> buffers, even those that are not part of ftrace. This file really has no 
> concept of a tracer. It simply stops writing to the ring buffer.
> 
> Things like the irq latency tracer is will still update its "max time" 
> when tracing is off, although the trace output will not be updated.
> 
> I could remove tracing_enabled, and move the tracing_on to trace.c, that 
> just calls the tracing_off of the ring buffer, and then the file will be 
> at the higher layer, and have a concept of the current tracer.

Sounds good to me. I think the highest utility is in the 'lightweight' 
tracing_on toggle - even though it does not do as thorough of a job of 
disabling a tracer as writing 0 to tracing_on.

I think we shouldnt even _try_ to provide a simple toggle for unregister: 
it is clear from looking at /debug/tracing/current_tracer that the current 
tracer is still active - so asking people to change that back to 'nop' 
will be rather intuitive IMO.

	Ingo
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