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

[Steven Rostedt]

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?

-- Steve