| Subject | Re: [PATCH tip/core/rcu 11/26] rcu: Exit RCU extended QS on user preemption | From | Peter Zijlstra <> | Date | Thu, 06 Sep 2012 19:13:11 +0200 |
| |
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?
|