Re: [RFC] Make need_resched() return true when rcu_urgent_qs requested

Paul E. McKenney
2018-07-06T17:09:43Z

On Fri, Jul 06, 2018 at 06:29:05PM +0200, Peter Zijlstra wrote:
> On Fri, Jul 06, 2018 at 03:53:30PM +0100, David Woodhouse wrote:
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index e4d4e60..89f5814 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -1616,7 +1616,8 @@ static inline int spin_needbreak(spinlock_t *lock)
> >
> > static __always_inline bool need_resched(void)
> > {
> > - return unlikely(tif_need_resched());
> > + return unlikely(tif_need_resched()) ||
> > + rcu_urgent_qs_requested();
> > }
>
> Instead of making need_resched() touch two cachelines, I think I would
> prefer adding resched_cpu() to rcu_request_urgent_qs_task().

I used to do something like this, but decided that whacking each holdout
CPU over the head ten times a second was a bit much.

> The preempt state is alread a bit complicated and shadowed in the
> preempt_count (on some architectures) adding additional bits to it like
> this is just asking for trouble.

How about a separate need_resched_rcu() that includes the extra cache
miss? Or open-coding the rcu_urgent_qs_requested()?

Thanx, Paul