On Sat, 2014-03-15 at 18:59 -0700, Paul E. McKenney wrote: > 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);
Hm. Since you only care about the case where your task is solo, how about do racy checks, 100% accuracy isn't required is it? Seems you wouldn't want to unconditionally do that in tight loops. -Mike -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
