linux-kernel - Re: [RFC PATCH 1/2] Marker probes in futex.c

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Message-Id: <1208501204.7115.60.camel@twins>
Date:	Fri, 18 Apr 2008 08:46:44 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	"Frank Ch. Eigler" <fche@...hat.com>
Cc:	prasad@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, mingo@...e.hu, mathieu.desnoyers@...ymtl.ca
Subject: Re: [RFC PATCH 1/2] Marker probes in futex.c

On Thu, 2008-04-17 at 18:02 -0400, Frank Ch. Eigler wrote:
> Peter Zijlstra <a.p.zijlstra@...llo.nl> writes:
> 
> > [...]
> >> If we were to log just the futex_ops, just as you had suggested,
> >> "Just log:
> >>  
> >>   futex: <uaddr> wait
> >>   futex: <uaddr> wakeup"
> >> [...]
> >> If you can specifically point me to information you think would be
> >> absolutely unnecessary, I can get them out of the trace_mark().
> >
> > I'm thinking everything is superflous; you're basically logging what
> > strace already gives you
> 
> But we don't want to run strace just for this stuff.  As you probably
> know, strace involves invasive user-space context-switching between
> the target and the tracer.
> 
> > except worse by encoding local variable names and exposing kernel
> > pointers.
> 
> The pointers are probably excessive, the and the names don't really
> matter.

Then what do we do when someone comes along and changes one of those
names; do we go around changing the markers and then requiring all tools
to change as well?

(And no this isn't far fetched; I'm thinking of changing fshared in the
near future).

Sounds like people will complain and generate back pressure against such
changes - something we should avoid. As soon as these markers place a
significant burden on code maintenance I'm against it.

>   What does matter is providing enough information for a
> problem diagnosis tool & person to reconstruct what the kernel must
> have been thinking when it did something noteworthy.

Sure, but then just make a strace like tracer and be done with it - no
need to pollute the futex code with that.

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