linux-kernel - Re: [PATCH 5/5] tracing: add saved_cmdlines file to show cached task comms

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Date:	Thu, 16 Apr 2009 18:05:39 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Avadh Patel <avadh4all@...il.com>
Subject: Re: [PATCH 5/5] tracing: add saved_cmdlines file to show cached
	task comms

On Thu, Apr 16, 2009 at 11:58:44AM -0400, Steven Rostedt wrote:
> 
> On Thu, 16 Apr 2009, Frederic Weisbecker wrote:
> 
> > On Wed, Apr 15, 2009 at 10:18:35PM -0400, Steven Rostedt wrote:
> > > From: Avadh Patel <avadh4all@...il.com>
> > > 
> > > Export the cached task comms to userspace. This allows user apps to translate
> > > the pids from a trace into their respective task command lines.
> > 
> > 
> > Hi,
> > 
> > I don't understand why this is needed. The pid is already resolved
> > to its task comm into the trace.
> 
> Nope, it is not. The trace buffer does not hold the comm. It is in an 
> internal cache within ftrace. This exports this table.


Ah ok, it's about the ring buffer direct fetching. I've only thought
about ftrace.


 
> > 
> > Or is there another reason?
> 
> If you were to perform a trace, and then stop it. Only the pids are in the 
> trace buffer. If those processes end, there's no way to find out what 
> process were attached to those pids. This table maps the pids in the 
> buffer to the comms saved in the cache. As long as you don't run another 
> trace, the cache will hold the pids in the trace.


Ok.


> 
> Note, this is for reading the binary data files.


And I missed that too.
Thanks for the explanations!


> -- Steve
> 

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