linux-kernel - Re: [PATCH 0/3] tracing/ftrace: ftrace

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Date:	Tue, 24 Feb 2009 18:25:30 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jason Baron <jbaron@...hat.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH 0/3] tracing/ftrace: ftrace_bprintk


* Jason Baron <jbaron@...hat.com> wrote:

> On Tue, Feb 24, 2009 at 06:16:18AM +0100, Frederic Weisbecker wrote:
> > Hi,
> > 
> > These three patches are part of a patchset posted by Lai Jiangshan in december 2008.
> > They introduce a binary version of ftrace_printk() called ftrace_bprintk()
> > 
> > While having the same goal: print a generic message entry into the ring buffer,
> > their approaches are very different.
> > 
> > - ftrace_printk() does the formatting job on tracing time, insert the whole resulting string
> >   into the ring buffer, and then the string is printed on output time without a lot of modifications.
> > 
> > - ftrace_bprintk() does no formatting on tracing time. Instead, it looks at the format string
> >   to find the types and the numbers of the arguments and directly stores them as-is into the
> >   ring-buffer. Then the format string is stored into the ring-buffer too, but only by its address,
> >   it is not copied. Then on output time only, the final string is formatted and sent to the user.
> >   This gives a result about as fast as a traditional tracer with fixed fields types, except that
> >   we can print random types and numbers of fields here.
> > 
> > 
> > The first patch adds the generic support for binary formatting.
> > The second adds the support for binary print types on ftrace
> > and the last introduces ftrace_bprintk() which supports safely the modules
> > by listening on the module loading/unloading notifier to keep track of
> > unwanted freed format strings.
> > 
> > Lai Jiangshan (3):
> >       add binary printf
> >       ftrace: infrastructure for supporting binary record
> >       ftrace: add ftrace_bprintk()
> > 
> 
> hi,
> 
> this seems like a really valuable feature....I'm just 
> wondering about a couple of things....
> 
> If the 'brpintk tracer' in trace/trace_bprintk.c is just being 
> used to set an enabled flag for printing out these binary 
> records, then are we better off with just an option flag in 
> the 'trace_options' file?
> 
> Second, can we somehow combine ftrace_printk() and 
> ftrace_bprintk(), so that a developer can just use one 
> interface? Perhaps, ftrace_printk calls ftrace_bprintk if 
> binary option flag is set, otherwise, it just outputs things 
> normally.

Well, ftrace_bprintk() seems to be a worthy and transparent 
replacement for ftrace_printk() to me. I.e. lets just use this 
as the new implementation for ftrace_printk().

Would there be any downsides of doing so? I dont see any.

	Ingo
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