Re: [RFC] Make need_resched() return true when rcu_urgent_qs requested

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Jul 11, 2018 at 03:23:45PM +0100, David Woodhouse wrote:
> 
> 
> On Mon, 2018-07-09 at 15:08 -0700, Paul E. McKenney wrote:
> > index f9c0ca2ccf0c..3350ece366ab 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -2839,6 +2839,15 @@ void rcu_check_callbacks(int user)
> >                 rcu_bh_qs();
> >         }
> >         rcu_preempt_check_callbacks();
> > +       /* The load-acquire pairs with the store-release setting to true. */
> > +       if (smp_load_acquire(this_cpu_ptr(&rcu_dynticks.rcu_urgent_qs))) {
> > +               /* Idle and userspace execution already are quiescent states. */
> > +               if (rcu_is_cpu_rrupt_from_idle() && !user) {
> 
> if (idle && !user) seems tautological and... illogical.
> 
> If I make it 'if (!rcu_is_cpu_rrput_from_idle() && !user)' it seems to
> work better. Ripping out my debugging printks now to check that's still
> true...

Right you are!  I will step away for a bit to put a paper bag over
my head...

> (Also, isn't userspace execution only a quiescent state if NO_HZ_FULL?)

Userspace execution is a quiescent state in all cases.  However, you
are quite right that NO_HZ_FULL makes a difference, namely, it allows
one CPU to reliably determine whether or not some other CPU is
currently executing either in userspace or in idle.

Without NO_HZ_FULL, CPUs can only detect their own userspace execution.
Which is what is happening here because rcu_check_callbacks() is being
invoked from the scheduling-clock interrupt, which is where the "user"
parameter comes from.

So the above code can reliably detect the usermode-execution quiescent
state because it is always running on the CPU in question.

							Thanx, Paul

> > +                       set_tsk_need_resched(current);
> > +                       set_preempt_need_resched();
> > +               }
> > +               __this_cpu_write(rcu_dynticks.rcu_urgent_qs, false);
> > +       }