Re: [RFC PATCH 03/10] ftrace: Drop the ftrace_profile_enabled checks in tracing hot path

[Frederic Weisbecker <fweisbec@gmail.com>]

On Mon, Jan 25, 2010 at 05:14:16PM -0500, Masami Hiramatsu wrote:
> Frederic Weisbecker wrote:
> > On Fri, Jan 22, 2010 at 07:34:51AM -0500, Mathieu Desnoyers wrote:
> >> * Steven Rostedt (rostedt@goodmis.org) wrote:
> >>> On Thu, 2010-01-21 at 23:09 -0500, Mathieu Desnoyers wrote:
> >>>> * Steven Rostedt (rostedt@goodmis.org) wrote:
> >>>
> >>>>> Hmm, interesting. Maybe something like that might work. But what if
> >>>>> CONFIG_PREEMPT is enabled but CONFIG_FREEZER is not?
> >>>>
> >>>> Then you may want to make the function tracer depend on CONFIG_FREEZER,
> >>>> but maybe Masami has other ideas ?
> >>>
> >>> egad no! This is just to help add guarantees to those that use the
> >>> function tracer that when the tracing is disabled, it is guaranteed that
> >>> no more tracing will be called by the function tracer. Currently,
> >>> nothing relies on this. But we may add cases that might need this.
> >>
> >> Yep, identifying tracer quiescent state can become handy.
> >>
> >>>
> >>> In fact, only those that need this requirement would need to do this
> >>> trick. Anyway, we could make those depend on CONFIG_FREEZER, but that
> >>> just seems to be a strange dependency.
> >>
> >> This makes me wonder (question for Masami)...
> >>
> >> static int __kprobes check_safety(void)
> >> {
> >>         int ret = 0;
> >> #if defined(CONFIG_PREEMPT) && defined(CONFIG_FREEZER)
> >>         ret = freeze_processes();
> >>         if (ret == 0) {
> >>                 struct task_struct *p, *q;
> >>                 do_each_thread(p, q) {
> >>                         if (p != current && p->state == TASK_RUNNING &&
> >>                             p->pid != 0) {
> >>                                 printk("Check failed: %s is running\n",p->comm);
> >>                                 ret = -1;
> >>                                 goto loop_end;
> >>                         }
> >>                 } while_each_thread(p, q);
> > 
> > 
> > 
> > How does that deal with kernel threads that don't freeze?
> 
> Hmm, right. It can't handle non-freezable kernel threads.
> 
> > Also freezing every processes seems a bit of a heavy thing for that.
> > Looks like a synchronize_tasks() would be really useful.
> 
> Sure :-)
> Maybe, I'd better remove booster support on preemptive kernel until then.


I don't know as I haven't looked deeper into check_safety(), but does the
fact we have non-freezable tasks break the assumptions that make
kprobes booster safe? If so then yeah, may be deactivate it for now.