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
