Re: [PATCH tip/core/rcu 11/20] sched,rcu: Make cond_resched() provide RCU quiescent state

[Michal Hocko]

On Mon 16-01-17 16:54:03, Paul E. McKenney wrote:
> On Mon, Jan 16, 2017 at 06:11:30PM +0100, Peter Zijlstra wrote:
> > On Sat, Jan 14, 2017 at 01:13:12AM -0800, Paul E. McKenney wrote:
> > > There is some confusion as to which of cond_resched() or
> > > cond_resched_rcu_qs() should be added to long in-kernel loops.
> > > This commit therefore eliminates the decision by adding RCU
> > > quiescent states to cond_resched().
> > 
> > Which would make: rcu_read_lock(); cond_resched(); rcu_read_unlock();
> > invalid under preemptible RCU. Is it already?
> 
> In theory, yes.  In practice, I just tested it with preemption and
> lockdep enabled, and it didn't complain.  If further testing finds
> complaints, we can either fix those uses (preferred) or revert
> this patch.
> 
> > > Warning: This is a prototype.  For example, it does not correctly
> > > handle Tasks RCU.  Which is OK for the moment, given that no one
> > > actually uses Tasks RCU yet.
> > 
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -4907,6 +4907,7 @@ int __sched _cond_resched(void)
> > >  		preempt_schedule_common();
> > >  		return 1;
> > >  	}
> > > +	rcu_all_qs();
> > >  	return 0;
> > >  }
> > 
> > Still not a real fan of this, it does make cond_resched() touch a bunch
> > more cachelines, also, I suppose that if we're going to do this, we
> > should make __cond_resched_lock() and __cond_resched_softirq() act
> > similarly.
> 
> Michal (now CCed) argues that having to distinguish between cond_resched()
> and cond_resched_rcu_qs() is overly burdensome.  Michal?

Yes, it is really not clear which one is meant to be in which context. I
really do not see which cond_resched should be turned intto
cond_resched_rcu_qs.

> Any thoughts on how we might remove this burden without the additional
> cache misses?  I will take another look as well to see what could make
> it lower cost.  There are probably ways...  Would it make sense to
> have RCU maintain a need-rcu_all_qs() flage in the same cacheline as
> the __preempt_count?  Perhaps throttling the writes to this flag from
> the RCU grace-period kthreads to once per 100 milliseconds or so?

Can the stall detector simply request rescheduling when it gets
dangerously close to the timeout?

-- 
Michal Hocko
SUSE Labs