linux-kernel - Re: [PATCH] new irq tracer

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Message-ID: <alpine.DEB.2.00.0902252040210.12697@gandalf.stny.rr.com>
Date:	Wed, 25 Feb 2009 20:41:33 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Jason Baron <jbaron@...hat.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Frank Ch. Eigler" <fche@...hat.com>, mingo@...e.hu,
	linux-kernel@...r.kernel.org, acme@...stprotocols.net,
	Dominique Toupin <dominique.toupin@...csson.com>
Subject: Re: [PATCH] new irq tracer


On Wed, 25 Feb 2009, Mathieu Desnoyers wrote:
> > > > 
> > > > Then use the function_graph tracer.
> > > > 
> > > 
> > > Sadly, the function tracer cannot be enabled on production systems.
> > > Therefore it's not a usable solution in the context where I need this.
> > > 
> > > Mathieu
> > 
> > 
> > Why?
> > If this is about the interrupt latency caused by stop_machine, I still don't understand.
> > This latency happens before the tracing (function filter which use patching), not during
> > tracing.
> > 
> 
> Given this scenario :
> 
> A telecommunication system runs, but the client notices something wrong.
> They call their service provider. The provider enables tracing
> _remotely_ on the _production system_ while it's _active in the field_.
> 
> Bam, those few milliseconds interrupt latencies become unacceptable.
> 
> Hopefully this scenario makes the use-case clearer. The problem is not
> that interrupt latencies would occur while tracing is on, but rather
> that it would happen on a running production system when switching
> tracing on. This is what is totally unacceptable for this use-case.
> 
> For more details about such requirements, I'm CCing Dominique Toupin
> from Ericsson who I'm sure would be happy to give more details about
> this if needed.

Hmm, so this system in the field is running Linux with the Real-Time 
Patch?  Because if it isn't it will suffer from millisecond latencies in 
normal operation.

-- Steve

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