Re: [PATCH 15/30] rcu: handle quiescent states for PREEMPT_RCU=n, PREEMPT_COUNT=y

[Joel Fernandes]

Hello Ankur and Paul,

On Mon, Feb 12, 2024 at 09:55:39PM -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 necessary: 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_AUTO=y, there can be configurations with (PREEMPT_COUNT=y,
> PREEMPT_RCU=n). This means that cond_resched() is a stub which does
> not provide for quiescent states via rcu_all_qs().
> 
> 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 26c79246873a..9b72e9d2b6fe 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -963,13 +963,16 @@ 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)))) {

I was wondering if it makes sense to even support !PREEMPT_RCU under
CONFIG_PREEMPT_AUTO.

AFAIU, this CONFIG_PREEMPT_AUTO series preempts the kernel on
the next tick boundary in the worst case, with all preempt modes including
the preempt=none mode.

Considering this, does it makes sense for RCU to be non-preemptible in
CONFIG_PREEMPT_AUTO=y? Because if that were the case, and a read-side critical
section extended beyond the tick, then it prevents the PREEMPT_AUTO preemption
from happening, because rcu_read_lock() would preempt_disable().

To that end, I wonder if CONFIG_PREEMPT_AUTO should select CONFIG_PREEMPTION
(or CONFIG_PREEMPT_BUILD, not sure which) as well because it does cause
kernel preemption. That then forces selection of CONFIG_PREEMPT_RCU as well.

thanks,

 - Joel







>  
>  		/*
>  		 * 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.
> +		 * 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
> -- 
> 2.31.1
>