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

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Message-ID: <alpine.DEB.2.00.0903161441210.27978@gandalf.stny.rr.com>
Date:	Mon, 16 Mar 2009 14:45:44 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.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


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.

The TRACE_FORMAT was made for things that will be recording string 
information anyway, and recording a string into the buffer via memcpy or a 
sprintf format (binary printk) doesn't make much difference.

Then trace points for entry and exit does not fall into that category, and 
should be represented by a TRACE_EVENT instead.

-- Steve

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