linux-kernel - Re: [to-be-posted-soon] Multiple handlers per marker

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Date:	Mon, 05 Nov 2007 14:46:59 -0800
From:	Mike Mason <mmlnx@...ibm.com>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
Cc:	ltt-dev@...fik.org, linux-kernel@...r.kernel.org,
	systemtap@...rces.redhat.com
Subject: Re: [to-be-posted-soon] Multiple handlers per marker

Mathieu Desnoyers wrote:
> * Mathieu Desnoyers (compudj@...stal.dyndns.org) wrote:
>> * Mathieu Desnoyers (mathieu.desnoyers@...ymtl.ca) wrote:
>>> * Mike Mason (mmlnx@...ibm.com) wrote:
>>>> Hi Mathieu,
>>>>
>>>> Are you aware of any working being done to allow multiple handlers to be 
>>>> attached to a marker?  Something like what kprobes allows.  I've started to 
>>>> look into this and don't want to duplicate efforts.
>>>>
>>> Nope, but I know we will have to address this.
>>>
>>> Something along the lines of walking an RCU list of function pointers,
>>> calling them.
>>>
>>> The only downside I see is that we will have to pass a va_list * instead
>>> of real va args. The could make the marker site a little bit bigger and
>>> will change the probe callback arguments.
>>>
>>> What do you think about these ideas ?
>>>
>>> If we can find a way to make the common case (only one probe connected)
>>> _ultra_ fast, and yet architecture independent, that would be awesome. A
>>> simple call is kind of hard to beat though.. So we may have to think
>>> about a design with :
>>>
>>> - One call at the marker site
>>> - if 1 probe is installed :
>>>   - If the format string is empty, connect a probe without va args.
>>>   - If the format string is not empty, connect a "stage 1" probe that takes
>>>     the va args, starts/ends the va_list and calls _one_ function (let's
>>>     call it "stage 2" probe), that takes va_list as parameter.
>>> - if more than 1 probe is installed :
>>>   - The stage 1 probe creates the va_list and passes it to each function
>>>     connected, iterated with an RCU list.
>>>
>>> What do you think ?
>>>
>>> Mathieu
>>>
>> I'm working on an implementation.
>>
> 
> It's ready for testing. Please grab
> http://ltt.polymtl.ca/lttng/patch-2.6.24-rc1-git13-lttng-0.10-pre18.tar.bz2
> patch name :
> 
> markers-support-multiple-probes.patch

This patch alone doesn't apply cleanly at all on 2.6.24-rc1-git14.  Are there other patches in this series I should apply first?

Mike

> 
> It still need to go through patchcheck.pl and some polishing, but it
> seems to work fine for me with multiple probes (the sample marker,
> sample probe and multiple instances of my lttng probes can
> connect/disconnect without problem).
> 
> Currently, the "connect/disconnect" and "arm/disarm" operations are
> separate. However, they could be merged. Any comment/preference on this?
> Being separate, a probe provider can wait until the very last moment
> before it activates its markers, with a minimalistic impact on the
> system, but it is not such a strong argument.
> 
> Mathieu
> 
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