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Date:	Sat, 13 Dec 2014 08:18:32 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Jiang Liu <jiang.liu@...ux.intel.com>, x86@...nel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Tony Luck <tony.luck@...el.com>,
	Joerg Roedel <joro@...tes.org>,
	Marc Zyngier <marc.zyngier@....com>,
	Yinghai Lu <yinghai@...nel.org>,
	Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [RFC PATCH 2/2] tracing: Add tracepoint_printk cmdline

On Sat, 13 Dec 2014 11:59:25 +0100
Borislav Petkov <bp@...en8.de> wrote:

> On Sat, Dec 13, 2014 at 12:50:37AM -0500, Steven Rostedt wrote:
> > 
> > Add the kernel command line tracepoint_printk option that will
> > have tracepoints that are active sent to printk().
> > 
> > Passing "tracepoint_printk" will activate this. To turn it off
> > the sysctl /proc/sys/kernel/tracepoint_printk can have '0' echoed
> > into it. Note, this only works if the cmdline option is used.
> > Echoing 1 into the sysctl file without the cmdline option will
> > have no affect.
> > 
> > Note, this is a dangerous option. Having high frequency
> > tracepoints send their data to printk() can possibly cause
> > a live lock.
> > 
> > Link: http://lkml.kernel.org/r/alpine.DEB.2.11.1412121539300.16494@nanos
> > 
> > Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> > Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> > ---
> >  Documentation/kernel-parameters.txt | 18 ++++++++++++++++++
> >  include/linux/ftrace.h              |  1 +
> >  kernel/sysctl.c                     |  7 +++++++
> >  kernel/trace/trace.c                | 17 +++++++++++++++++
> >  kernel/trace/trace.h                |  1 +
> >  kernel/trace/trace_events.c         | 32 ++++++++++++++++++++++++++++++++
> >  6 files changed, 76 insertions(+)
> > 
> > diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> > index 1d09eb37c562..d81f464a7358 100644
> > --- a/Documentation/kernel-parameters.txt
> > +++ b/Documentation/kernel-parameters.txt
> > @@ -3500,6 +3500,24 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
> >  			See also Documentation/trace/ftrace.txt "trace options"
> >  			section.
> >  
> > +	tracepoint_printk[FTRACE]
> 
> Damn, this is long and most likely nasty to type on some dingy box when
> you're trying to debug stuff. Can we shorten it?
> 
> trace_printk

No this is already taken.

> tp_printk

Actually, I was thinking about shortening it to tp_printk.

> tp_print
> tp_pr
> ...
> 
> Last one is my favourite - we should call it ToiletPaper_Print :-)

What you need when your kernel is in the crapper.

-- Steve

> 
> > +			Have the tracepoints sent to printk as well as the
> > +			tracing ring buffer. This is useful for early boot up
> > +			where the system hangs or reboots and does not give the
> > +			option for reading the tracing buffer or performing a
> > +			ftrace_dump_on_oops.
> 

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