decoding of multi-byte nops in dtrace

[Andriy Gapon]

on 10/06/2012 20:56 Fabian Keil said the following:
> Andriy Gapon <avg at FreeBSD.org> wrote:
> 
>> on 10/06/2012 16:27 Fabian Keil said the following:
>>> Andriy Gapon <avg at 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 at r500 /usr/src $sudo dtrace -ln fbt::: | grep -c entry
>>> 23395
>>> fk at r500 /usr/src $sudo dtrace -ln fbt::: | grep -c return
>>> 16739
>>>
>>> With the patch (and updated kernel sources):
>>>
>>> fk at r500 ~ $sudo dtrace -ln fbt::: | grep -c entry
>>> 23409
>>> fk at r500 ~ $sudo dtrace -ln fbt::: | grep -c return
>>> 19879
>>
>> Interesting observations, thank you.
>> Do you use -O2 or higher optimization for kernel/modules build?
> 
> Yes, I kept the default -O2.
> 
>> I use only -O1.
> 
> With -O1 (and your patch) I get:
> 
> fk at r500 ~ $sudo dtrace -ln fbt::: | grep -c entry
> 23421
> fk at r500 ~ $sudo dtrace -ln fbt::: | grep -c return
> 22621
> 
>> 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.
> 
> I looked at a couple of the functions that still lack return
> probes and the ones I looked at don't seem to belong into these
> categories.
> 
> For example I get no return probes for g_eli_crypto_decrypt()
> and g_eli_crypto_encrypt(). Both return the return code of
> g_eli_crypto_cipher() for which I get a return probe.

I don't have GELI in kernel, but it looks like an instance of well-known tail
call optimization issue.  Although I assumed that GCC wouldn't apply it at -O1.
Perhaps you use a module that was built with -O2.

-- 
Andriy Gapon