Re: [PATCH tip/core/rcu 11/26] rcu: Exit RCU extended QS on user preemption

[Peter Zijlstra <peterz@infradead.org>]

On Thu, 2012-09-06 at 19:02 +0200, Peter Zijlstra wrote:
> On Thu, 2012-08-30 at 14:05 -0700, Paul E. McKenney wrote:
> > From: Frederic Weisbecker <fweisbec@gmail.com>
> > 
> > When exceptions or irq are about to resume userspace, if
> > the task needs to be rescheduled, the arch low level code
> > calls schedule() directly.
> > 
> > At that time we may be in extended quiescent state from RCU
> > POV: the exception is not anymore protected inside
> > rcu_user_exit() - rcu_user_enter() and the irq has called
> > rcu_irq_exit() already.
> > 
> > Create a new API schedule_user() that calls schedule() inside
> > rcu_user_exit()-rcu_user_enter() in order to protect it. Archs
> > will need to rely on it now to implement user preemption safely.
> 
> > ---
> >  kernel/sched/core.c |    7 +++++++
> >  1 files changed, 7 insertions(+), 0 deletions(-)
> > 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 0bd599b..e841dfc 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -3463,6 +3463,13 @@ asmlinkage void __sched schedule(void)
> >  }
> >  EXPORT_SYMBOL(schedule);
> >  
> > +asmlinkage void __sched schedule_user(void)
> > +{
> > +	rcu_user_exit();
> > +	schedule();
> > +	rcu_user_enter();
> > +}
> 
> 
> OK, so colour me unconvinced.. why are we doing this?
> 
> Typically when we call schedule nr_running != 1 (we need current to be
> running and a possible target to switch to).
> 
> So I'd prefer to simply have schedule() disable all this adaptive tick
> nonsense and leave it at that.

In fact, the only way to get here is through ttwu(), which would have
done the nr_running increment and should have disabled all this adaptive
stuff.

So again,.. why?