From: "Rafael J. Wysocki" <rjw@sisk.pl>
To: Steven Rostedt <rostedt@goodmis.org>
Subject: Re: ftraced and suspend to ram
Date: Thu, 21 Aug 2008 20:37:10 +0200
User-Agent: KMail/1.9.6 (enterprise 20070904.708012)
Cc: Nigel Cunningham <nigel@nigel.suspend2.net>, Pavel Machek <pavel@ucw.cz>,
       LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@elte.hu>,
       Andrew Morton <akpm@linux-foundation.org>,
       Linus Torvalds <torvalds@linux-foundation.org>
References: <alpine.DEB.1.10.0808211139420.3947@gandalf.stny.rr.com> <200808212015.05768.rjw@sisk.pl> <alpine.DEB.1.10.0808211423410.678@gandalf.stny.rr.com>
In-Reply-To: <alpine.DEB.1.10.0808211423410.678@gandalf.stny.rr.com>
MIME-Version: 1.0
Content-Type: text/plain;
  charset="iso-8859-1"
Content-Transfer-Encoding: 7bit
Content-Disposition: inline
Message-Id: <200808212037.11775.rjw@sisk.pl>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2382
Lines: 54

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.

Rafael
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/