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

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

On Mon, Jul 09, 2018 at 07:50:54PM +0100, David Woodhouse wrote:
> 
> 
> On Mon, 2018-07-09 at 09:34 -0700, Paul E. McKenney wrote:
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 51919985f6cf..33b0a1ec0536 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -2496,6 +2496,10 @@ void rcu_check_callbacks(int user)
> >  {
> >  	trace_rcu_utilization(TPS("Start scheduler-tick"));
> >  	raw_cpu_inc(rcu_data.ticks_this_gp);
> > +	if (smp_load_acquire(this_cpu_ptr(&rcu_dynticks.rcu_urgent_qs)) &&
> > +	   !is_idle_task(current))
> > +		set_tsk_need_resched(current);
> 
> OK, so this will make KVM (and various other code) see that
> need_resched() is true, and they'll call cond_resched() or something
> else that might not actually schedule another task, but will at least
> end up in rcu_all_qs()...
> 
> > +	__this_cpu_write(rcu_dynticks.rcu_urgent_qs, false);
> 
> ... which bails out immediately and does nothing, because that's set to
> false?
> 
> Am I missing something?

If this is the idle task, RCU will detect that as a quiescent state via
its dyntick-idle mechanism.  In which case, there is no point in leaving
.rcu_urgent_qs being true.

If this is not the idle task, the scheduler will invoke
rcu_note_context_switch(), which will in turn invoke rcu_sched_qs(),
rcu_preempt_qs(), or rcu_qs(), depending on kernel version and
configuration.  This will happen independently of .rcu_urgent_qs, so it
is OK to set .rcu_urgent_qs to false.  And doing so reduces the overhead
of the next cond_resched().

I may end up using rcu_is_cpu_rrupt_from_idle() instead of is_idle_task()
at some point, but the former eases backporting.  And the only difference
is if someone has a long loop within an _rcuidle tracepoint used in the
idle loop, and where that loop check need_resched().  Which currently
seems to be the empty set.

And I should treat interruption of a usermode task the same as that of an
idle task.  In the PREEMPT case, ->rcu_read_lock_nesting better be zero
(lockdep would have complained), so the quiescent state will be reported.
In the !PREEMPT case, user=1 directly causes reporting of a quiescent
state.

So here are the possible code paths when .rcu_urgent_qs is set to true:

1.	A context switch will record the quiescent state and clear
	.rcu_urgent_qs.  (The failure to do the clearing in current -rcu
	for PREEMPT builds is a performance bug that I need to fix.)

2.	A cond_resched() will cause rcu_all_qs() to be invoked, which
	will record a quiescent state and clear .rcu_urgent_qs.

3.	With the patch below, a scheduling-clock interrupt of a
	non-idle non-userspace task will force a reschedule, which
	will result in #1 above happening.

However, I should avoid setting .rcu_urgent_qs to false when it is
already false, shouldn't I?

So how about the following instead?

I am doing some light testing and will let you know how that goes.

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 51919985f6cf..c3b688c7127a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2496,6 +2496,15 @@ void rcu_check_callbacks(int user)
 {
 	trace_rcu_utilization(TPS("Start scheduler-tick"));
 	raw_cpu_inc(rcu_data.ticks_this_gp);
+	/* The load-acquire pairs with the store-release setting to true. */
+	if (smp_load_acquire(this_cpu_ptr(&rcu_dynticks.rcu_urgent_qs))) {
+		/* Idle already is a quiescent state. */
+		if (!is_idle_task(current) && !user) {
+			set_tsk_need_resched(current);
+			set_preempt_need_resched();
+		}
+		__this_cpu_write(rcu_dynticks.rcu_urgent_qs, false);
+	}
 	rcu_flavor_check_callbacks(user);
 	if (rcu_pending())
 		invoke_rcu_core();