On Tue, Nov 21, 2023 at 05:52:09PM -0500, Steven Rostedt wrote:
> On Tue, 21 Nov 2023 14:26:33 -0800
> "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote:
>
> > On Tue, Nov 21, 2023 at 04:38:34PM -0500, Steven Rostedt wrote:
> > > On Tue, 21 Nov 2023 13:14:16 -0800
> > > "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote:
> > >
> > > > On Tue, Nov 21, 2023 at 09:30:49PM +0100, Peter Zijlstra wrote:
> > > > > On Tue, Nov 21, 2023 at 11:25:18AM -0800, Paul E. McKenney wrote:
> > > > > > #define preempt_enable() \
> > > > > > do { \
> > > > > > barrier(); \
> > > > > > if (!IS_ENABLED(CONFIG_PREEMPT_RCU) && raw_cpu_read(rcu_data.rcu_urgent_qs) && \
> > > > > > (preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK | HARDIRQ_MASK | NMI_MASK) == PREEMPT_OFFSET) &&
> > > > > > !irqs_disabled()) \
> > >
> > > Could we make the above an else case of the below if ?
> >
> > Wouldn't that cause the above preempt_count() test to always fail?
>
> preempt_count_dec_and_test() returns true if preempt_count() is zero, which
> happens only if NEED_RESCHED is set, and the rest of preempt_count() is not
> set. (NEED_RESCHED bit in preempt_count() is really the inverse of
> NEED_RESCHED). Do we need to call rcu_all_qs() when we call the scheduler?
> Isn't scheduling a quiescent state for most RCU flavors?
>
> I thought this was to help move along the quiescent states without added
> cond_resched() around, which has:
>
> int __sched __cond_resched(void)
> {
> if (should_resched(0)) {
> preempt_schedule_common();
> return 1;
> }
> /*
> * In preemptible kernels, ->rcu_read_lock_nesting tells the tick
> * whether the current CPU is in an RCU read-side critical section,
> * so the tick can report quiescent states even for CPUs looping
> * in kernel context. In contrast, in non-preemptible kernels,
> * RCU readers leave no in-memory hints, which means that CPU-bound
> * processes executing in kernel context might never report an
> * RCU quiescent state. Therefore, the following code causes
> * cond_resched() to report a quiescent state, but only when RCU
> * is in urgent need of one.
> */
> #ifndef CONFIG_PREEMPT_RCU
> rcu_all_qs();
> #endif
> return 0;
> }
>
> Where if we schedule, we don't call rcu_all_qs().
True enough, but in this __cond_resched() case we know that we are in
an RCU quiescent state regardless of what should_resched() says.
In contrast, with preempt_enable(), we are only in a quiescent state
if __preempt_count_dec_and_test() returns true, and even then only if
interrupts are enabled.
> 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?
Thanx, Paul
