linux-kernel - Re: Kernel marker has no performance impact on ia64.

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Message-ID: <20080612221055.GA8142@Krystal>
Date:	Thu, 12 Jun 2008 18:10:55 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Frank Ch. Eigler" <fche@...hat.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Hideo AOKI <haoki@...hat.com>, mingo@...e.hu,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: Kernel marker has no performance impact on ia64.

* Peter Zijlstra (peterz@...radead.org) wrote:
> On Thu, 2008-06-12 at 12:43 -0400, Frank Ch. Eigler wrote:
> 
> > This is another reason why markers are a nice solution.  They allow
> > passing of actual useful values: not just the %p pointers but the most
> > interesting derived values (e.g., prev->pid). 
> 
> Useful to whoem? stap isn't the holy grail of tracing and certainly not
> the only consumer of trace points, so restricting the information to
> just what stap needs is actually making trace points less useful.
> 

LTTng also currently relies on the markers identifying useful data and
ideally won't contain any tracepoint-specific code to extract subfields
from structures.

By the way, a point that should be clarified is that putting a 
"prev->pid" field in a marker does not make this variable live unless
the marker is activated. When disabled, the marker site jumps over the
stack setup, including any pointer dereference.

Mathieu

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Mathieu Desnoyers
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