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
