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

Paul E. McKenney
2017-01-18T07:18:27Z

On Tue, Jan 17, 2017 at 01:11:46PM +0100, Michal Hocko wrote:
> 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.

Thank you!!!

The other complication is that under CONFIG_PREEMPT=y, _cond_resched()
is an empty function. That would be one reason why use of cond_resched()
wasn't always giving RCU the quiescent states that it needs. And that
is a problem with this patch, which I therefore need to defer to 4.12.

That aside, the reason I am reluctant to use the need-resched approach
except as an emergency measure is that the way I have to set that bit
remotely involves IPIs.

But don't get me wrong, it is extremely useful as an emergency meaure.
I am just trying to get cond_resched() to help on a non-emergency basis.

Thanx, Paul