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SubjectRe: ftraced and suspend to ram

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.

>
> 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,

-- Steve



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