linux-kernel - Re: ftrace: trace_pipe

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Message-ID: <1293165326.22802.411.camel@gandalf.stny.rr.com>
Date:	Thu, 23 Dec 2010 23:35:26 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	David Sharp <dhsharp@...gle.com>
Cc:	linux-kernel@...r.kernel.org, Michael Rubin <mrubin@...gle.com>
Subject: Re: ftrace: trace_pipe_raw interface broken

On Thu, 2010-12-23 at 15:06 -0800, David Sharp wrote:
> On Thu, Dec 23, 2010 at 7:57 AM, Steven Rostedt <rostedt@...dmis.org> wrote:

> > Not quite intended, but not something to worry about either. We mask off
> > the 30 bits to determine the size.
> 
> in trace-cmd you mean?

Yeah, but if you find that strange, I guess we could mask it off too.

> 
> >> By my count that leaves only one mystery: why are we seeing this extra
> >> page at the beginning with pre-overflow data?
> >
> > If you did not reset the buffer, there's a chance that the writer is on
> > the reader page. The reader page is always outside the ring buffer, but
> > it points into the ring buffer. If this occurs, then you will get the
> > reader page data, plus the rest of the ring buffer (which is the full
> > size you asked for).
> 
> Okay, that's a surprising behavior to me, but resetting the buffer
> seems like a fine workaround.

Getting a little more trace than you asked for seems like a good thing
to me ;-)

> 
> btw, corroborating this is that reading the formatted output of
> "trace" also shows the page of pre-overflow data.

Yep, that's already known.

-- Steve


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