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

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Date:	Thu, 28 Jan 2016 15:47:05 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	rostedt <rostedt@...dmis.org>,
	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, Jan 28, 2016 at 02:38:16PM +0000, Mathieu Desnoyers wrote:
> ----- On Jan 28, 2016, at 8:57 AM, 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.
> 
> One item I have on my todo list is to allow mapping
> the tracer buffers (lttng in my case) onto RAM that
> persists across reboots/kexec using dax and the pmem
> driver. Since the original system is clearly inactive
> after a reboot, we can read the buffers from memory
> without caring about synchronization.

I would expect the new kernel to use the same buffer for its tracing,
right? After all, there's no distinction between the old and new kernel
except a reboot.

In which case, my kernels often start babbling the moment I boot. So I
need to take care not to scribble the old data.

> That would be one possible way of handling your
> snapshot-of-buggy-kernel-trace-buffers use-case.

Now I wonder if my ipmipower -r is a warn reset :-)

(I'm assuming the pmem/dax stuff would simply use regular RAM to back
this stuff)