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

[Mathieu Desnoyers <compudj@krystal.dyndns.org>]

* Frederic Weisbecker (fweisbec@gmail.com) wrote:
> 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.
> 

In the case of check_safety, it's not a bug per se if a task happens to
be non freezable. freeze_processes() will likely return a non-zero
value, and the whole check_safety will therefore return that value, so
standard breakpoints will be used instead.

But that doesn't fit with the function graph tracer requirements.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
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