linux-kernel - Re: ftraced and suspend to ram

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Date:	Thu, 21 Aug 2008 21:59:36 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Nigel Cunningham <nigel@...el.suspend2.net>,
	Pavel Machek <pavel@....cz>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: ftraced and suspend to ram

On Thursday, 21 of August 2008, Rafael J. Wysocki wrote:
> On Thursday, 21 of August 2008, Steven Rostedt wrote:
> > 
> > On Thu, 21 Aug 2008, Rafael J. Wysocki wrote:
> > 
> > > On Thursday, 21 of August 2008, Steven Rostedt wrote:
> > > > 
> > > > 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.
> > > > 
> > > > Again, this daemon method makes resume from suspend to ram reboot instead 
> > > > of resuming. Now, I'm asking the s2r gurus, what did I miss? Do I need to 
> > > > add a "NO_FREEZE" flag or something to the "ftraced" kernel thread?
> > > > 
> > > > Just asking for some advice.
> > > 
> > > If I'm not mistaken, it'll probably suffice to make it freezable, so that it
> > > doesn't run while the system is suspending and resuming.  Would that be
> > > acceptable?
> > 
> > Does it not freeze by default.
> 
> No, it doesn't.  Kernel threads are not freezable by default
> 
> > > Please tell me where exactly the ftraced source code is located.
> > 
> > It's in Linus's latest git tree.
> > 
> > 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?

Thanks,
Rafael

---
 kernel/trace/ftrace.c |    5 +++++
 1 file changed, 5 insertions(+)

Index: linux-2.6/kernel/trace/ftrace.c
===================================================================
--- linux-2.6.orig/kernel/trace/ftrace.c
+++ linux-2.6/kernel/trace/ftrace.c
@@ -796,8 +796,13 @@ 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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