Re: [PATCH] Linux Kernel Markers

[Satoshi Oshima <soshima@redhat.com>]

Mathieu Desnoyers wrote:
> * Vara Prasad (prasadav@us.ibm.com) wrote:
>> Martin Bligh wrote:
>>
>>> [...]
>>> Depends what we're trying to fix. I was trying to fix two things:
>>>
>>> 1. Flexibility - kprobes seem unable to access all local variables etc
>>> easily, and go anywhere inside the function. Plus keeping low overhead
>>> for doing things like keeping counters in a function (see previous
>>> example I mentioned for counting pages in shrink_list).
>>>
>> Using tools like systemtap on can consult DWARF information and put 
>> probes in the middle of the function and access local variables as well, 
>> that is not the real problem. The issue here is compiler doesn't seem to 
>> generate required DWARF information in some cases due to optimizations.  
>> The other related problem is when there exists debug information, the 
>> way to specify the breakpoint location is using line number which is not 
>> maintainable, having a marker solves this problem as well. Your proposal 
>> still doesn't solve the need for markers if i understood correctly.
>>
> 
> His implementation makes a heavy use of a marker mechanism : this is exactly
> what permits to create the instrumented objects from the same source code, but
> with different #defines.

Djprobes don't depend on markers. Actually, markers help to find the
safe place to probe, but they are not necessary. At least, instructions
that are more than 4 byte are probable.

As Vara pointed out, we are developing the tools that find the
safe place for djprobes. 


Satoshi OSHIMA