Re: [PATCH v2 4/6] rcu: handle quiescent states for PREEMPT_RCU=n, PREEMPT_COUNT=y

[Sebastian Andrzej Siewior]

On 2024-11-06 12:17:56 [-0800], Ankur Arora wrote:
> With PREEMPT_RCU=n, cond_resched() provides urgently needed quiescent
> states for read-side critical sections via rcu_all_qs().
> One reason why this was needed, was lacking preempt-count, the tick
> handler has no way of knowing whether it is executing in a read-side
> critical section or not.
> 
> With PREEMPT_LAZY=y, there can be configurations with PREEMPT_COUNT=y,
> PREEMPT_RCU=n, where cond_resched() is a stub that does not provide
> quiescent states via rcu_all_qs().

With PREEMPT_LAZY=y && PREEMPT_DYNAMIC=n we get PREEMPT_COUNT=y and
PREEMPT_RCU=n. In this configuration cond_resched() is an empty stub and
does not provide quiescent states via rcu_all_qs(). PREEMPT_RCU=y
provides this information via rcu_read_unlock() and its nesting counter.

> So, use the availability of preempt_count() to report quiescent states
> in rcu_flavor_sched_clock_irq().

Okay. You might also want to update the cond_resched() comment,
	s@In preemptible kernels, ->rcu_read_lock_nesting@
	  In PREEMPT_RCU kernels, ->rcu_read_lock_nesting@

Reviewed-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>

> Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>

Sebastian