cond_resched() and RCU CPU stall warnings

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

So I have been tightening up rcutorture a bit over the past year.
The other day, I came across what looked like a great opportunity for
further tightening, namely the schedule() in rcu_torture_reader().
Why not turn this into a cond_resched(), speeding up the readers a bit
and placing more stress on RCU?

And boy does it increase stress!

Unfortunately, this increased stress sometimes shows up in the form of
lots of RCU CPU stall warnings.  These can appear when an instance of
rcu_torture_reader() gets a CPU to itself, in which case it won't ever
enter the scheduler, and RCU will never see a quiescent state from that
CPU, which means the grace period never ends.

So I am taking a more measured approach to cond_resched() in
rcu_torture_reader() for the moment.

But longer term, should cond_resched() imply a set of RCU
quiescent states?  One way to do this would be to add calls to
rcu_note_context_switch() in each of the various cond_resched() functions.
Easy change, but of course adds some overhead.  On the other hand,
there might be more than a few of the 500+ calls to cond_resched() that
expect that RCU CPU stalls will be prevented (to say nothing of
might_sleep() and cond_resched_lock()).

Thoughts?

(Untested patch below, FWIW.)

							Thanx, Paul

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

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b46131ef6aab..994d2b0fd0b2 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4075,6 +4075,9 @@ int __sched _cond_resched(void)
 		__cond_resched();
 		return 1;
 	}
+	preempt_disable();
+	rcu_note_context_switch(smp_processor_id());
+	preempt_enable();
 	return 0;
 }
 EXPORT_SYMBOL(_cond_resched);