linux-kernel - Re: [PATCH] ftrace: mcountrecord.pl for arm

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Message-ID: <alpine.DEB.1.10.0811210823460.6447@gandalf.stny.rr.com>
Date:	Fri, 21 Nov 2008 08:31:14 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
cc:	Jim Radford <radford@...vanix.net>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linuxppc-dev@...abs.org, Paul Mundt <lethal@...ux-sh.org>,
	Matt Fleming <mjf@...too.org>, Sam Ravnborg <sam@...nborg.org>,
	linux-arm-kernel@...ts.arm.linux.org.uk,
	Abhishek Sagar <sagar.abhishek@...il.com>
Subject: Re: [PATCH] ftrace: mcountrecord.pl for arm


On Fri, 21 Nov 2008, Russell King - ARM Linux wrote:

> On Thu, Nov 20, 2008 at 02:11:49PM -0800, Jim Radford wrote:
> > Ingo and Steven,
> > 
> > Here's an updated version of the arch/arm changes for dynamic ftrace
> > based on top of your latest tip/master.
> 
> Excuse me if I'm rather confused, but...
> 
> When ftrace for ARM was originally merged, neither linux-arm-kernel
> nor myself were copied with the patches.  Now, I'm being sent updates
> to code that I've no understanding of and haven't seen before.
> 
> I mean, yes, it's nice to be copied with patches which are relevent.
> It would've been even nicer to have been copied with the patches adding
> ftrace in the first place, so people knew something about it and were
> aware of the changes.
> 
> It seems to me like there's been a total breakdown of communication
> when ftrace was initially merged...

Yes I totally agree that in the beginning there was a breakdown of
communication. I myself just learned of the ARM port.

> 
> So, questions: has ftrace actually been tested on ARM at all?  Has it
> been reviewed?  Which ARM platforms has it been tried on?  How stable
> is it?  How has it been implemented on ARM?  Does it rely on any CPU
> specific behaviour?
> 
> Looking at the git history, ftrace was merged via Ingo, so I assume
> that Ingo has some understanding of this code.  So, for the time being
> if these are urgent updates, I suggest that updates go through Ingo's
> tree rather than mine.

I would suggest that they at least get an ACK from you. The original
code should have too.

> 
> And looking at arch/arm/kernel/ftrace.c, it's incompatible with Thumb2
> which we've been working towards supporting.  What about SMP?  ARM is
> a SMP capable architecture now, and I see no locking in there - what
> I do see is static data with pointers to it being returned to other
> code... Yuck.

Some of this code will be redesigned in 29. But as for the locking, this 
code is run under kstop_machine. Which means that even on SMP 
architectures, this acts like a UP box.  Some of the code can be run 
outside of kstop_machine, but it is protected by locks in the module code.
I'll take a look at the ftrace.c arm code and see if there's any problems 
with it. I wrote the x86 version as well as the coming PowerPC port.

Thanks,

-- Steve

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