linux-kernel - Re: [RFC][BUG] tracer: Fails to work

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Message-ID: <20160128091854.45d0ac2b@gandalf.local.home>
Date:	Thu, 28 Jan 2016 09:18:54 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC][BUG] tracer: Fails to work

On Thu, 28 Jan 2016 14:57:33 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, Jan 28, 2016 at 01:38:00PM +0000, Mathieu Desnoyers wrote:
> 
> > Thoughts ?  
> 
> So ideally dumping the trace data would not depend on any of that,
> because I can break it all :-)
> 
> Not being able to access the trace data completely and utterly defeats
> the purpose of having a tracer in the first place.
> 

The solution is to use trace_pipe. That's for live tracing (or in this
case, tracing when the system gets corrupted). I may update the tracing
documentation to note this.

The trace file preserves the data it reads, and disables the ring
buffer before reading it to keep the iterator from being corrupted. But
that requires a somewhat working system (not a CPU being stuck). Hence,
use trace_pipe for your debugging. The only thing you'll lose is the
header that trace provides.

-- Steve