Re: [RFC PATCH 48/86] rcu: handle quiescent states for PREEMPT_RCU=n

[Paul E. McKenney]

On Tue, Nov 21, 2023 at 07:12:32PM -0500, Steven Rostedt wrote:
> On Tue, 21 Nov 2023 16:01:24 -0800
> "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote:
> 
> > 
> > > I stand by that being in the else statement. It looks like that would keep
> > > the previous work flow.  
> > 
> > Ah, because PREEMPT_NEED_RESCHED is zero when we need to reschedule,
> > so that when __preempt_count_dec_and_test() returns false, we might
> > still be in an RCU quiescent state in the case where there was no need
> > to reschedule.  Good point!
> > 
> > In which case...
> > 
> > #define preempt_enable() \
> > do { \
> > 	barrier(); \
> > 	if (unlikely(preempt_count_dec_and_test())) \
> > 		__preempt_schedule(); \
> > 	else if (!sched_feat(FORCE_PREEMPT) && \
> > 		 (preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK | HARDIRQ_MASK | NMI_MASK) == PREEMPT_OFFSET) && \
> > 		 !irqs_disabled()) \
> > ) \
> > 			rcu_all_qs(); \
> > } while (0)
> > 
> > Keeping rcu_all_qs() pretty much as is.  Or some or all of the "else if"
> > condition could be pushed down into rcu_all_qs(), depending on whether
> > Peter's objection was call-site object code size, execution path length,
> > or both.  ;-)
> > 
> > If the objection is both call-site object code size and execution path
> > length, then maybe all but the preempt_count() check should be pushed
> > into rcu_all_qs().
> > 
> > Was that what you had in mind, or am I missing your point?
> 
> Yes, that is what I had in mind.
> 
> Should we also keep the !IS_ENABLED(CONFIG_PREEMPT_RCU) check, which makes
> the entire thing optimized out when PREEMPT_RCU is enabled?

I substituted the !sched_feat(FORCE_PREEMPT) for this because as I
understand it, sites currently using CONFIG_PREEMPT_DYNAMIC=y (which is
the default) and booting with preempt=none will currently have their
grace periods helped by cond_resched(), so likely also need help,
perhaps also from preempt_enable().

							Thanx, Paul