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

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Date:	Fri, 16 Jan 2009 09:32:32 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Chris Mason <chris.mason@...cle.com>
Subject: Re: [PATCH 0/2] ftrace: updates to tip


On Fri, 16 Jan 2009, Ingo Molnar wrote:

> 
> * Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > Ingo,
> > 
> > The first patch is critical, and needs to stay with trace_output.c Not 
> > that critical since trace_output.c is not in mainline yet.
> > 
> > The second patch gives the ability to stack trace functions. I've been 
> > leery about adding this and still keep it a separate option from the 
> > "stacktrace" that already exists. This is because when enabled with no 
> > filtering, the lag between typing and seeing what is typed can be up to 
> > 10 seconds or more.
> 
> Btw., is this true even if frame pointers are enabled? When frame pointers 
> are disabled we scan the full kernel stack => that can be quite slow if 
> every kernel function is traced ;-)

After making my latest changes, I see a half sec lag. I don't know
why it was so slow before. But I do have all the debug features enabled
which would also slow things down quite a bit (lockdep, et. al)

> 
> > I made the function stack trace an option attached to the function 
> > tracer, so it must be enabled after the function tracer has been set. 
> > This still needs to be updated in ftrace.txt.
> 
> maybe we could drive this via the filter API? Something like:
> 
>    echo "*btrfs*:stacktrace" >> set_filter_functions
> 
> Would automatically mean that those functions will all generate 
> stacktraces too. Note how safe this API is by default: the filter is used 
> for a narrow scope of functions anwyay. To get it for all kernel functions 
> one would have to do:
> 
>    echo "*:stacktrace" >> set_filter_functions
> 
> Which one cannot do accidentally.
> 
> What do you think?

Now that I see only a 1/2 sec lag, do you still think this is necessary?

Maybe I should go back and see why it was so bad before?

But I do notice that not all functions produce a valid stack trace. Maybe 
it would be better to add that api :-?

-- Steve

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