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Date: Sun, 10 Jun 2012 19:38:35 +0300
From: Andriy Gapon <avg@FreeBSD.org>
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To: Fabian Keil <freebsd-listen@fabiankeil.de>
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Cc: freebsd-hackers@FreeBSD.org
Subject: Re: decoding of multi-byte nops in dtrace
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on 10/06/2012 16:27 Fabian Keil said the following:
> Andriy Gapon <avg@FreeBSD.org> wrote:
> 
>> It seems that the $subj is missing :-)
>> In my environment that causes many functions to not have fbt return probe,
>> because function body decoding fails before 'ret' is found.
>>
>> Here is my attempt at fixing the problem:
>> http://people.freebsd.org/~avg/fbt-nop.patch
>> Reviews and suggestions are welcome.
> 
> The patch seems to reduce the number of missing
> fbt return probes by about 50% for me.
> 
> Without the patch:
> 
> fk@r500 /usr/src $sudo dtrace -ln fbt::: | grep -c entry
> 23395
> fk@r500 /usr/src $sudo dtrace -ln fbt::: | grep -c return
> 16739
> 
> With the patch (and updated kernel sources):
> 
> fk@r500 ~ $sudo dtrace -ln fbt::: | grep -c entry
> 23409
> fk@r500 ~ $sudo dtrace -ln fbt::: | grep -c return
> 19879

Interesting observations, thank you.
Do you use -O2 or higher optimization for kernel/modules build?
I use only -O1.

Here are some stats from my system:
$ dtrace -ln fbt::: | fgrep -c entry
16876
$ dtrace -ln fbt::: | fgrep -c return
16729

So, 147 functions without return probe.
>From a quick look at them they all seem to really never return.  Either they are
noreturn type such panic, or functions that always call the functions of the
first type, or functions with endless loops in them such as top level functions
of many system threads.
-- 
Andriy Gapon