Re: [PATCH 4/7] rcu: handle quiescent states for PREEMPT_RCU=n, PREEMPT_COUNT=y

[Sebastian Andrzej Siewior]

On 2024-10-09 09:54:08 [-0700], 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().

PREEMPT_LAZY selects PREEMPT_BUILD which selects PREEMPTION which in
turn selects PREEMPT_RCU. So this is not a valid combination. Do you
have a different tree than I do? Because maybe I am missing something.

> So, use the availability of preempt_count() to report quiescent states
> in rcu_flavor_sched_clock_irq().
> 
> Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
> ---
>  kernel/rcu/tree_plugin.h | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 1c7cbd145d5e..da324d66034b 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -974,13 +974,16 @@ static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
>   */
>  static void rcu_flavor_sched_clock_irq(int user)
>  {
> -	if (user || rcu_is_cpu_rrupt_from_idle()) {
> +	if (user || rcu_is_cpu_rrupt_from_idle() ||
> +	     (IS_ENABLED(CONFIG_PREEMPT_COUNT) &&
> +	      !(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)))) {

couldn't you use a helper preemptible()?

>  		/*
>  		 * Get here if this CPU took its interrupt from user
> -		 * mode or from the idle loop, and if this is not a
> -		 * nested interrupt.  In this case, the CPU is in
> -		 * a quiescent state, so note it.
> +		 * mode, from the idle loop without this being a nested
> +		 * interrupt, or while not holding a preempt count (but
> +		 * with PREEMPT_COUNT=y. In this case, the CPU is in a
> +		 * quiescent state, so note it.
>  		 *
>  		 * No memory barrier is required here because rcu_qs()
>  		 * references only CPU-local variables that other CPUs

Sebastian