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

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Date:	Wed, 17 Nov 2010 15:18:25 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	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>,
	Tom Zanussi <tzanussi@...il.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:
> 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.
> 
> So I think it would be wise to make this change sooner rather than
> later. 

Also, I see a lot of overlap with the dynamic probes stuff which needs
the help of magic dwarves to come up with the right data to gather.

Merging the dynamic tracepoint and filter stuff would be nice, there's
no way you can express next->prio without the help of these short
buggers with large axes.

The trouble is that the dynamic tracepoint stuff is privileged for good
reasons, try next+0x1000000 and you're out in the woods, priv. only
filters otoh just doesn't sound too hot.

Another nasty thing is that you actually need to have these dwarves
present, which means having the -debug package installed.


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