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

[Peter Zijlstra]

On Mon, Jul 09, 2018 at 01:42:48PM -0700, Paul E. McKenney wrote:
> On Mon, Jul 09, 2018 at 09:35:38PM +0100, David Woodhouse wrote:
> > 
> > 
> > On Mon, 2018-07-09 at 13:34 -0700, Paul E. McKenney wrote:
> > > 
> > > So here are the possible code paths when .rcu_urgent_qs is set to true:
> > > 
> > > 1.      A context switch will record the quiescent state and clear
> > >         .rcu_urgent_qs.  (The failure to do the clearing in current -rcu
> > >         for PREEMPT builds is a performance bug that I need to fix.)
> > 
> > What if there's nothing else runnable and there is no actual context
> > switch?
> 
> The scheduler invokes rcu_note_context_switch() before looking to see
> if there really will or won't be a context switch.

Correct. Just getting to __schedule() means we can schedule and thus is
a valid point for RCU to progress. Even if we then end up selecting the
very same task and not switching at all.