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

Paul E. McKenney
2018-07-09T20:40:39Z

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.

I am sure that Peter will correct me if I am confused on this point. ;-)

Thanx, Paul

> > 2.      A cond_resched() will cause rcu_all_qs() to be invoked, which
> >         will record a quiescent state and clear .rcu_urgent_qs.
> >
> > 3.      With the patch below, a scheduling-clock interrupt of a
> >         non-idle non-userspace task will force a reschedule, which
> >         will result in #1 above happening.