linux-kernel - Re: [PATCH 0/11] LTTng-core (basic tracing infrastructure) 0.5.108

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Date:	Fri, 15 Sep 2006 10:57:17 -0700
From:	Andrew Morton <akpm@...l.org>
To:	fche@...hat.com (Frank Ch. Eigler)
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, karim@...rsys.com,
	Roman Zippel <zippel@...ux-m68k.org>,
	Tim Bird <tim.bird@...sony.com>, Ingo Molnar <mingo@...e.hu>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	linux-kernel@...r.kernel.org,
	Christoph Hellwig <hch@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tom Zanussi <zanussi@...ibm.com>, ltt-dev@...fik.org,
	Michel Dagenais <michel.dagenais@...ymtl.ca>
Subject: Re: [PATCH 0/11] LTTng-core (basic tracing infrastructure) 0.5.108

On 15 Sep 2006 13:08:29 -0400
fche@...hat.com (Frank Ch. Eigler) wrote:

> Alan Cox <alan@...rguk.ukuu.org.uk> writes:
> 
> > [...]
> > > 
> > >    		prepare_arch_switch(rq, next);
> > > +		TRACE_SCHEDCHANGE(prev, next);
> > >    		prev = context_switch(rq, prev, next);
> > >    		barrier();
> > 
> > The gdb debug data lets you find each line and also the variable
> > assignments (except when highly optimised in some cases). [...]
> 
> Unfortunately, variables and even control flow are quite regularly
> made non-probe-capable by modern gcc.  Statement boundaries and
> variables are not preserved.  There is an arms race within gcc to both
> improve code optimization and its own "reverse-engineering" debugging
> data generation, and the former is always ahead.
> 
> The end result is that there are many spots that we'd like to probe in
> systemtap, but can't place exactly or extract all the data we'd like.
> Really.

Useful info, thanks.

> There are also spots that for other reasons cannot tolerate a fully
> dynamic kprobes-style probe:
> 
> - where 1000-cycle int3-dispatching overheads too high

Is that still true of the recent kprobes "boosting" changes?

> - in low-level code such as fault handling or locking, that, if probed
>   dynamically, could entail infinite regress
> - debugging information may not be available
> 
> This is the reason why I'm in favour of some lightweight event-marking
> facility: a way of catching those points where dynamic probing is not
> sufficiently fast or dependable.

OK.

> > [...]
> > All we appear to lack is systemtap ability to parse debug data so it can
> > be told "trace on line 9 of sched.c and record rq and next"
> 
> Actually:
> 
> #! stap
> probe kernel.function("*@...nel/sched.c:9") { printf("%p %p", $rq, $next) }
> 

Really.  That's impressive progress.
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