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

[Steven Rostedt]

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().

I stand by that being in the else statement. It looks like that would keep
the previous work flow.

-- Steve