linux-kernel - Re: [PATCH] ftrace: disable tracing for suspend to ram

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Date:	Wed, 27 Aug 2008 23:27:25 +0200
From:	Marcin Slusarz <marcin.slusarz@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, Pavel Machek <pavel@...e.cz>,
	Ingo Molnar <mingo@...e.hu>,
	Nigel Cunningham <nigel@...el.suspend2.net>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] ftrace: disable tracing for suspend to ram

On Wed, Aug 27, 2008 at 09:14:40AM -0400, Steven Rostedt wrote:
> 
> I've been painstakingly debugging the issue with suspend to ram and 
> ftraced. The 2.6.28 code does not have this issue, but since the mcount 
> recording is not going to be in 27, this must be solved for the ftrace 
> daemon version.
> 
> The resume from suspend to ram would reboot because it was triple 
> faulting. Debugging further, I found that calling the mcount function 
> itself was not an issue, but it would fault when it incremented 
> preempt_count. preempt_count is on the tasks info structure that is on the 
> low memory address of the task's stack.  For some reason, it could not 
> write to it. Resuming out of suspend to ram does quite a lot of funny 
> tricks to get to work, so it is not surprising at all that simply doing a 
> preempt_disable() would cause a fault.
> 
> Thanks to Rafael for suggesting to add a "while (1);" to find the place in 
> resuming that is causing the fault. I would place the loop somewhere in 
> the code, compile and reboot and see if it would either reboot (hit the 
> fault) or simply hang (hit the loop).  Doing this over and over again, I 
> narrowed it down that it was happening in enable_nonboot_cpus.
> 
> At this point, I found that it is easier to simply disable tracing around 
> the suspend code, instead of searching for the particular function that 
> can not handle doing a preempt_disable.
> 
> This patch disables the tracer as it suspends and reenables it on resume.
> 
> I tested this patch on my Laptop, and it can resume fine with the patch.
> 
> Signed-off-by: Steven Rostedt <srostedt@...hat.com>

You can add my:
Tested-by: Marcin Slusarz <marcin.slusarz@...il.com>

Thanks!

Marcin
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