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

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Date:	Thu, 26 Feb 2009 07:37:18 -0500
From:	"Dominique Toupin" <dominique.toupin@...csson.com>
To:	"Steven Rostedt" <rostedt@...dmis.org>,
	"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>
Subject: RE: [PATCH] new irq tracer


In many cases we don't use Linux real-time, we have many systems that
are soft-real-time an non real-time Linux is good enough.
 

> -----Original Message-----
> From: Steven Rostedt [mailto:rostedt@...dmis.org] 
> Sent: 25-Feb-09 20:42
> To: Mathieu Desnoyers
> Cc: Frederic Weisbecker; Jason Baron; Masami Hiramatsu; 
> KOSAKI Motohiro; Peter Zijlstra; Frank Ch. Eigler; 
> mingo@...e.hu; linux-kernel@...r.kernel.org; 
> acme@...stprotocols.net; Dominique Toupin
> 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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