On Thu, Apr 06, 2017 at 10:14:25AM -0400, Steven Rostedt wrote:
> On Wed, 5 Apr 2017 21:15:15 -0700
> "Paul E. McKenney" <paul...@linux.vnet.ibm.com> wrote:
> \> > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> > > index 8efd9fe..28e3019 100644
> > > --- a/kernel/trace/ftrace.c
> > > +++ b/kernel/trace/ftrace.c
> > > @@ -2808,18 +2808,28 @@ static int ftrace_shutdown(struct ftrace_ops
> > > *ops, int command)
> > > * callers are done before leaving this function.
> > > * The same goes for freeing the per_cpu data of the per_cpu
> > > * ops.
> > > - *
> > > - * Again, normal synchronize_sched() is not good enough.
> > > - * We need to do a hard force of sched synchronization.
> > > - * This is because we use preempt_disable() to do RCU, but
> > > - * the function tracers can be called where RCU is not watching
> > > - * (like before user_exit()). We can not rely on the RCU
> > > - * infrastructure to do the synchronization, thus we must do it
> > > - * ourselves.
> > > */
> > > if (ops->flags & (FTRACE_OPS_FL_DYNAMIC | FTRACE_OPS_FL_PER_CPU)) {
> > > + /*
> > > + * We need to do a hard force of sched synchronization.
> > > + * This is because we use preempt_disable() to do RCU, but
> > > + * the function tracers can be called where RCU is not watching
> > > + * (like before user_exit()). We can not rely on the RCU
> > > + * infrastructure to do the synchronization, thus we must do it
> > > + * ourselves.
> > > + */
> > > schedule_on_each_cpu(ftrace_sync);
> >
> > Great header comment on ftrace_sync(): "Yes, function tracing is rude."
> > And schedule_on_each_cpu() looks like a great workqueue gatling gun! ;-)
> >
> > > +#ifdef CONFIG_PREEMPT
> > > + /*
> > > + * When the kernel is preeptive, tasks can be preempted
> > > + * while on a ftrace trampoline. Just scheduling a task on
> > > + * a CPU is not good enough to flush them. Calling
> > > + * synchronize_rcu_tasks() will wait for those tasks to
> > > + * execute and either schedule voluntarily or enter user space.
> > > + */
> > > + synchronize_rcu_tasks();
> > > +#endif
> >
> > How about this to save a line?
> >
> > if (IS_ENABLED(CONFIG_PREEMPT))
> > synchronize_rcu_tasks();
>
> Ah, this works as gcc optimizes it out. Otherwise I received a compile
> error with synchronize_rcu_tasks() not defined. But that's because I
> never enabled CONFIG_TASKS_RCU.
Should I define a synchronize_rcu_tasks() that does BUG() for this case?
> > One thing that might speed this up a bit (or might not) would be to
> > doe the schedule_on_each_cpu() from a delayed workqueue. That way,
> > if any of the activity from schedule_on_each_cpu() involved a voluntary
> > context switch (from a cond_resched() or some such), then
> > synchronize_rcu_tasks() would get the benefit of that context switch.
> >
> > You would need a flush_work() to wait for that delayed workqueue
> > as well, of course.
>
> This is a very slow path, I'm not too interested in making it complex
> to speed it up.
Makes sense to me!
> > Not sure whether it is worth it, but figured I should pass it along.
> >
> > > arch_ftrace_trampoline_free(ops);
> > >
> > > if (ops->flags & FTRACE_OPS_FL_PER_CPU)
> > > @@ -5366,22 +5376,6 @@ void __weak arch_ftrace_update_trampoline(struct
> > > ftrace_ops *ops)
> > >
> > > static void ftrace_update_trampoline(struct ftrace_ops *ops)
> > > {
> > > -
> > > -/*
> > > - * Currently there's no safe way to free a trampoline when the kernel
> > > - * is configured with PREEMPT. That is because a task could be preempted
> > > - * when it jumped to the trampoline, it may be preempted for a long time
> > > - * depending on the system load, and currently there's no way to know
> > > - * when it will be off the trampoline. If the trampoline is freed
> > > - * too early, when the task runs again, it will be executing on freed
> > > - * memory and crash.
> > > - */
> > > -#ifdef CONFIG_PREEMPT
> > > - /* Currently, only non dynamic ops can have a trampoline */
> > > - if (ops->flags & FTRACE_OPS_FL_DYNAMIC)
> > > - return;
> > > -#endif
> > > -
> > > arch_ftrace_update_trampoline(ops);
> > > }
> >
> > Agreed, straightforward patch!
>
> Great, I'll start making it official then.
Sounds very good!
Thanx, Paul
