linux-kernel - Re: [Patch 2/2] tracepoints for softirq entry/exit

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Message-ID: <20090316192817.GC11878@Krystal>
Date:	Mon, 16 Mar 2009 15:28:17 -0400
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Jason Baron <jbaron@...hat.com>, mingo@...e.hu,
	linux-kernel@...r.kernel.org, acme@...stprotocols.net,
	fweisbec@...il.com, fche@...hat.com, peterz@...radead.org
Subject: Re: [Patch 2/2] tracepoints for softirq entry/exit - tracepoints

* Steven Rostedt (rostedt@...dmis.org) wrote:
> 
> On Mon, 16 Mar 2009, Steven Rostedt wrote:
> 
> > 
> > On Mon, 16 Mar 2009, Mathieu Desnoyers wrote:
> > > > > 
> > > > > The softirq tracepoints are a good idea indeed (I have similar ones in
> > > > > the LTTng tree). My main concern is about the fact that you output the
> > > > > softirq name in plain text to the trace buffers. I would rather prefer
> > > > > to save only the softirq (h-vec) into the trace and dump the mapping
> > > > > (h-vec) to name only once, so we can save precious trace bytes.
> > > > 
> > > > The TP_FMT is only used by those tracers that want to use it. Any tracer 
> > > > can still hook directly to the trace point and do what every they want.
> > > > 
> > > > -- Steve
> > > > 
> > > 
> > > By doing so, you are removing the ability to use the TP_FMT information
> > > to perform high-speed system-wide tracing. I thought the goal was to
> > > create a unified buffering, but sadly I don't see the high-speed
> > > requirements being part of that plan.
> > 
> > TP_FMT has nothing to do with the unified buffering. The unified buffer 
> > does not even know about it. But if you want high-speed event tracing, 
> > that is what the TRACE_EVENT was created for.
> 
> Here's an example:
> 
> The "event tracing" uses the format field to show those events for the 
> hook in the sched switching, and wake ups.
> 
> The wake up tracer on the other hand, does not care about the format, it 
> only cares about having a hook where a a task is woken up, and where it 
> gets scheduled in, and perhaps events in between. But it uses its own 
> formatting to do the output.
> 
> -- Steve
> 

If I understand you correctly, the format string is only useful to the
text-output tracer ? Why can't it be used to identify both text and
binary events ?

And remember that from my perspective, information is only useful if
available for system-wide tracing. Specialized tracers come as a subset
of system-wide tracing anyway when the latter is implemented with the
proper hooks.

Mathieu


-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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