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Date: Mon, 14 Sep 2009 20:55:41 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Masami Hiramatsu <mhiramat@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@elte.hu>,
       lkml <linux-kernel@vger.kernel.org>,
       systemtap <systemtap@sources.redhat.com>,
       DLE <dle-develop@lists.sourceforge.net>,
       Jim Keniston <jkenisto@us.ibm.com>,
       Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
       Andi Kleen <ak@linux.intel.com>, Christoph Hellwig <hch@infradead.org>,
       "Frank Ch. Eigler" <fche@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
       Jason Baron <jbaron@redhat.com>, "K.Prasad" <prasad@linux.vnet.ibm.com>,
       Lai Jiangshan <laijs@cn.fujitsu.com>, Li Zefan <lizf@cn.fujitsu.com>,
       Peter Zijlstra <peterz@infradead.org>,
       Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
       Tom Zanussi <tzanussi@gmail.com>
Subject: Re: [PATCH tracing/kprobes 4/7] tracing/kprobes: Add event
	profiling support
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On Mon, Sep 14, 2009 at 12:54:24PM -0400, Masami Hiramatsu wrote:
>>> I'd like to have a dispatcher function and flags internally :)
>>
>>
>> You mean kprobes that could support multiple probes?
>> That would be a nice solution IMHO...
>
> Yeah, actually kprobes could support multiple probes on the
> same point. But kprobe structure has many extensions which
> kprobe-tracer doesn't need, e.g. post_handler/break_handler,
> opcode, arch sprcific instructions.
> Kretprobe consumes more memories for storing return points :(.
>
> Thus, if we know there are two functions to be called on the
> same probe point, I think it is better to have a dispatcher.
> (Especially, in this case, we can call fixed functions, so
> it's easier way.)
>


Yeah, you could union the post_handler with profile_handler
or something like that.

It depends if kprobes may need one day to support an undeterminate
number of probes.

Also, is the post_handler called at the same location than the normal
probe?
And is a post handler called even if there is no normal handler?


There might be some of such factors that would force you to handle a
lot of corner cases, things that you wouldn't need to worry about
if you just had to maintain a simple rcu list of probes to call.