linux-kernel - Re: [PATCH] Linux Kernel Markers

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Date:	Tue, 19 Sep 2006 18:27:35 -0400
From:	Satoshi Oshima <soshima@...hat.com>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
CC:	Vara Prasad <prasadav@...ibm.com>,
	Martin Bligh <mbligh@...gle.com>, prasanna@...ibm.com,
	Andrew Morton <akpm@...l.org>,
	"Frank Ch. Eigler" <fche@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Paul Mundt <lethal@...ux-sh.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Jes Sorensen <jes@....com>, Tom Zanussi <zanussi@...ibm.com>,
	Richard J Moore <richardj_moore@...ibm.com>,
	Michel Dagenais <michel.dagenais@...ymtl.ca>,
	Christoph Hellwig <hch@...radead.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	William Cohen <wcohen@...hat.com>, ltt-dev@...fik.org,
	systemtap@...rces.redhat.com, Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] Linux Kernel Markers

Mathieu Desnoyers wrote:
> * Vara Prasad (prasadav@...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

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