linux-kernel - Re: [PATCH 2/2] tracing/function-graph-tracer: use the more lightweight local clock

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Message-Id: <1236238213.5330.10111.camel@laptop>
Date:	Thu, 05 Mar 2009 08:30:13 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] tracing/function-graph-tracer: use the more
 lightweight local clock

On Thu, 2009-03-05 at 02:19 +0100, Frederic Weisbecker wrote:

> It takes 1 ms to execute while tracing.
> Considering my frequency is 250 Hz, it means 1/4 of the system is used
> on timer interrupt while tracing.
> 
> For now the hang is fixed, but not the awful latency. And I'm just too frightened
> to test it on 1000 Hz.
> 
> But I plan to add a kind of watchdog to check how many time we spent inside an
> interrupt while graph tracing.
> By checking this time against the current Hz value, I could decide to abort the tracing
> for all irq.

That would basically render the thing useless :-(

Is it specifically function_graph that is so expensive? If so, is that
because of the function exit hook?

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