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

[Paul E. McKenney]

On Tue, Nov 07, 2023 at 01:57:34PM -0800, Ankur Arora wrote:
> cond_resched() is used to provide urgent quiescent states for
> read-side critical sections on PREEMPT_RCU=n configurations.
> This was necessary because lacking preempt_count, there was no
> way for the tick handler to know if we were executing in RCU
> read-side critical section or not.
> 
> An always-on CONFIG_PREEMPT_COUNT, however, allows the tick to
> reliably report quiescent states.
> 
> Accordingly, evaluate preempt_count() based quiescence 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 |  3 ++-
>  kernel/sched/core.c      | 15 +--------------
>  2 files changed, 3 insertions(+), 15 deletions(-)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index f87191e008ff..618f055f8028 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -963,7 +963,8 @@ 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() ||
> +	    !(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK))) {

This looks good.

>  		/*
>  		 * Get here if this CPU took its interrupt from user
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index bf5df2b866df..15db5fb7acc7 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8588,20 +8588,7 @@ int __sched _cond_resched(void)
>  		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

But...

Suppose we have a long-running loop in the kernel that regularly
enables preemption, but only momentarily.  Then the added
rcu_flavor_sched_clock_irq() check would almost always fail, making
for extremely long grace periods.  Or did I miss a change that causes
preempt_enable() to help RCU out?

							Thanx, Paul

>  	return 0;
>  }
>  EXPORT_SYMBOL(_cond_resched);
> -- 
> 2.31.1
>