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

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Message-ID: <20090224174604.GG5765@nowhere>
Date:	Tue, 24 Feb 2009 18:46:05 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Jason Baron <jbaron@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, 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

On Tue, Feb 24, 2009 at 12:17:18PM -0500, Jason Baron 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? 


This tracer does a bit more, it keeps track of the string formats used
by a module which uses ftrace_bprintk().
So for now, the use of this facility can't be decoupled from its tracer.
In fact, I will try to move this module safety into tracing core so that
any tracer will be able to use it.

 
> 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.
> 
> thanks,
> 
> -Jason 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 

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