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Subject: Re: ftrace introduces instability into kernel 2.6.27(-rc2,-rc3)
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Jeremy Fitzhardinge <jeremy@goop.org>
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Date: Wed, 20 Aug 2008 11:17:28 +1000
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Cc: Nick Piggin <nickpiggin@yahoo.com.au>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
	linux-kernel@vger.kernel.org,
	Steven Rostedt <rostedt@goodmis.org>, linuxppc-dev@ozlabs.org,
	Steven Rostedt <srostedt@redhat.com>, Scott Wood <scottwood@freescale.com>,
	Eran Liberty <liberty@extricom.com>
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On Tue, 2008-08-19 at 16:58 -0700, Jeremy Fitzhardinge wrote:
> Benjamin Herrenschmidt wrote:
> >> Ok, there are two cases where it's ok :
> >>
> >> 1 - in stop_machine, considering we are not touching code executed in
> >> NMI handlers.
> >> 2 - when using my replace_instruction_safe() which uses a temporary
> >> breakpoint when doing the instruction replacement.
> >>
> >> In those cases you could use text_poke_early().
> >>     
> >
> > Note that vmap/vunmap will be very slow.
> >   
> 
> Don't we have Nick's speedups now?

Not sure what speedups this is, but it's stlil not something you want to
do a lot ... setting up and tearing down MMU mappings isn't something
I'd like to see happening on a per-mcount basis.

Ben