linux-kernel - Re: ftraced and suspend to ram

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Date:	Fri, 22 Aug 2008 06:46:49 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Nigel Cunningham <nigel@...el.suspend2.net>,
	Pavel Machek <pavel@....cz>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: ftraced and suspend to ram


* Rafael J. Wysocki <rjw@...k.pl> wrote:

> > > The code in question is the ftraced() function in 
> > > kernel/trace/ftrace.c
> > 
> > Thanks, I'll have a look in a while.
> 
> Can you try the appended patch, please?

makes sense - i've applied it to tip/tracing/urgent, see the tidied up 
commit below.

It should be no big issue not being able to trace across suspend+resume 
- and that restriction will go away with Steve's build-time based mcount 
patching mechanism in v2.6.28.

	Ingo

------------->
>From 0e556695ddc8eebf6f6dd86bb0c4911b2b90c12a Mon Sep 17 00:00:00 2001
From: Rafael J. Wysocki <rjw@...k.pl>
Date: Thu, 21 Aug 2008 21:59:36 +0200
Subject: [PATCH] ftrace: fix ftraced and suspend to ram

Steven Rostedt observed:

> In latest 2.6.27(git) enabling dynamic ftrace makes resume from a suspend
> to ram reboot instead of resuming. Queued for 2.6.28 is a new method of
> recording mcount callers at compile time that does not have this issue.
>
> But the new method is still too "green" to be pulled into 27, so the old
> ftraced (daemon method) needs to be fixed for 27.
>
> The way dynamic ftrace works with the daemon method is this. On boot up
> the mcount function simply returns. When ftrace is initialized, it calls
> kstop_machine to modify the mcount function to call another function
> called "ftrace_record_ip". This new function will record in a preallocated
> hash (allocated by the ftrace initializer) all the callers of mcount. A
> check is made to see if the caller has already been put into the hash, and
> if so, it is not recorded again.
>
> Later on a kernel thread ftraced is created. This kernel thread wakes up
> once a second and checks to see if any new functions were added to the
> hash. If so, it then calls kstop_machine and modifies those callers to
> mcount into nops.

It will suffice to make it freezable, so that it doesn't run while the
system is suspending and resuming.

Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 kernel/trace/ftrace.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 639e16c..49f4c3f 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -819,8 +819,13 @@ static int ftraced(void *ignore)
 {
 	unsigned long usecs;
 
+	set_freezable();
+
 	while (!kthread_should_stop()) {
 
+		if (try_to_freeze())
+			continue;
+
 		set_current_state(TASK_INTERRUPTIBLE);
 
 		/* check once a second */
--
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