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

[Michal Hocko]

On Tue 17-01-17 04:05:13, Paul E. McKenney wrote:
> On Tue, Jan 17, 2017 at 11:51:41AM +0100, Michal Hocko wrote:
> > 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?
> 
> It is quite possible that half of the stall timeout would be a better
> choice than my 100 milliseconds, but either way, there would be need
> for a flag or some such.

E.g. set_tsk_need_resched() on the task currently running on a cpu which
is preventing the rcu grace period for too long?

That would only require change to the stall detector and the cond_resched
could be left alone completely.
-- 
Michal Hocko
SUSE Labs