Date: Thu, 12 Jun 2008 12:43:18 -0400
From: "Frank Ch. Eigler" <fche@redhat.com>
To: Masami Hiramatsu <mhiramat@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
       Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
       Hideo AOKI <haoki@redhat.com>, mingo@elte.hu,
       linux-kernel@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>
Subject: Re: Kernel marker has no performance impact on ia64.
Message-ID: <20080612164318.GB17814@redhat.com>
References: <48447052.3030300@redhat.com> <1212445965.6269.22.camel@lappy.programming.kicks-ass.net> <20080602232135.GA20173@Krystal> <1212618449.19205.35.camel@lappy.programming.kicks-ass.net> <20080604232241.GA8488@Krystal> <1212653539.19205.47.camel@lappy.programming.kicks-ass.net> <20080612135319.GB22348@Krystal> <1213280823.31518.114.camel@twins> <20080612155332.GA16658@redhat.com> <48514BE3.3000506@redhat.com>
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Hi -

On Thu, Jun 12, 2008 at 12:16:35PM -0400, Masami Hiramatsu wrote:
> [...]
> > Think this through.  How should systemtap (or another user-space
> > separate-compiled tool like lttng) do this exactly?
> > [...]
> > (d) or another way?
> 
> use a lookup table. we can expect that the marking points which
> regularly inserted in the upstream kernel are stable(not so
> frequently change). In that case, we can easily maintain
> a lookup table which has pairs of format strings like as
> "sched_switch(struct task_struct * next, struct task_struct * prev)":"next %p prev %p"
> out of tree. Thus, you can use the printf-style format parser.

That's an interesting idea, but errors in this table would themselves
only be caught at C compilation time.  Worse, it does nothing helpful
for actually pulling out the next/prev fields of interest.  Remember,
real tracing users don't care so much about the task_struct pointers,
but about observable things like PIDs.  Systemtap would be back to the
debuginfo or C-header-guessing/parsing job (or embedded-C, yuck).

This is another reason why markers are a nice solution.  They allow
passing of actual useful values: not just the %p pointers but the most
interesting derived values (e.g., prev->pid).  And they do this
*efficiently* - in out-of-line code that imposes no measurable
overhead in the normal case..


- FChE
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