linux-kernel - Re: HW breakpoints perf

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Date:	Tue, 19 Jan 2010 15:40:06 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Joshua Pincus <joshua.pincus@...il.com>,
	"K.Prasad" <prasad@...ux.vnet.ibm.com>, peterz@...radead.org,
	paulus@...ba.org, acme@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: HW breakpoints perf_events request

On Mon, Jan 18, 2010 at 12:35:59PM +0100, Andi Kleen wrote:
> On Mon, Jan 18, 2010 at 12:04:07PM +0100, Frederic Weisbecker wrote:
> > I don't quite understand what signals are masked here,
> 
> ptrace reports all signals to the tracer and only
> delivers them on PTRACE_CONT. "Masking" is probably
> not the right term for it.


Ah, ok.

> > actually I'm not sure what is the true problem with ptrace.
> > Is it because a breakpoint in a thread is going to stop
> > all thread in the process until the parent handles the
> > signal?
> 
> I didn't think ptrace stopped all threads by its own,
> it just stops the current one and also only until 
> the problem is reported. And for a break point you
> typically want (in fact need to) to stop until it is handled.


Yeah, ok.

 
> > Anyway, although I first suggested extending perf, with
> > more thoughts I now agree that perf should keep doing what
> > it does currently (profiling) and not trying to become an
> > messy mix of a profiler, debugger, etc...
> 
> I find it surprising that you say that -- my
> impression is that whatever gets proposed recently: 
> error injection, testing, error handling, any other event 
> someone proposed perf as the answer.


Event injection is not off boundaries. This is still
in the topic of profiling/tracing.

Not sure what you mean. My point was that perf should
keep being an event performance observation tool, and
that controlling the execution flow of a process is not
the role of perf.

 
> 
> > What about extending ptrace to support a new type of
> > breakpoint debugging interface?
> 
> There's this utrace interface, but it seems to be more
> a in kernel layer with some "we can't make up our mind what
> the interface to the user looks like" and a lot of
> overengineering in interfaces thrown in.
> 
> ptrace in its current form is somewhat messy, but for
> me it seems there isn't anything better. So yes extending
> it would seem like a good idea.
> 
> A good starting point might be the debug store interfaces
> that got recently added to ptrace.


I need to look at this then. Thanks.

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