linux-kernel - Re: [ANNOUNCE] New utility: 'trace'

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Message-ID: <1290008001.1921.14.camel@elnicho>
Date:	Wed, 17 Nov 2010 09:33:21 -0600
From:	Tom Zanussi <tzanussi@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Ted Ts'o <tytso@....edu>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Jason Baron <jbaron@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Christoph Hellwig <hch@....de>,
	Pekka Enberg <penberg@...nel.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [ANNOUNCE] New utility: 'trace'

On Wed, 2010-11-17 at 15:11 +0100, Peter Zijlstra wrote:
> On Wed, 2010-11-17 at 15:00 +0100, Ingo Molnar wrote:
> > > But yes, it is functional.
> > 
> > I suspect that is what matters mostly - unless you think that it's impossible to 
> > have a sane implementation of it, if the users come. 
> 
> I'm not sure, I raised the point several times, and I think Steve and
> Tom though it was possible to implement sanely, but I'm not sure if it
> was expression invariant (the fix that is).
> 
> It would be very good to have a definite answer on that.
> 
> The idea was to not let the filter engine work on the trace data (once
> its gathered) but on the trace argument right at the beginning of the
> tracepoint callchain, since some of the trace data is an expression of
> the trace argument (say next->prio instead of next), the trace
> expression wouldn't stay invariant, you'd have to write a different
> filter for the same effect.
> 

IIRC, I think the conclusion we came to was that it could be done
mechanically if for example the right-hand-side of an assignment in
TP_fast_assign() only involved a simple variable assignment, but as
Steve pointed out, some assignments are more complicated than that.

For example, in the sched_switch tracepoint assignments:

	__entry->prev_prio      = prev->prio;
        __entry->prev_state     = __trace_sched_switch_state(prev);

so the prev_prio should be able to be tested 'in-line' but the
prev_state would require a temporary buffer to write the value into
before doing the test as mentioned by Steve.  At which point you're no
further ahead (in that case) than the current situation...

Tom

> So I think it would be wise to make this change sooner rather than
> later.


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