Re: [PATCH 4/4] rcu/nocb: Make shrinker to iterate only NOCB CPUs

[Joel Fernandes <joel@joelfernandes.org>]

On Wed, Mar 22, 2023 at 08:44:56PM +0100, Frederic Weisbecker wrote:
> Callbacks can only be queued as lazy on NOCB CPUs, therefore iterating
> over the NOCB mask is enough for both counting and scanning. Just lock
> the mostly uncontended barrier mutex on counting as well in order to
> keep rcu_nocb_mask stable.
> 

Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

thanks,

 - Joel


> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> ---
>  kernel/rcu/tree_nocb.h | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index a3dc7465b0b2..185c0c9a60d4 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1319,13 +1319,21 @@ lazy_rcu_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
>  	int cpu;
>  	unsigned long count = 0;
>  
> +	if (WARN_ON_ONCE(!cpumask_available(rcu_nocb_mask)))
> +		return 0;
> +
> +	/*  Protect rcu_nocb_mask against concurrent (de-)offloading. */
> +	mutex_lock(&rcu_state.barrier_mutex);
> +
>  	/* Snapshot count of all CPUs */
> -	for_each_possible_cpu(cpu) {
> +	for_each_cpu(cpu, rcu_nocb_mask) {
>  		struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
>  
>  		count +=  READ_ONCE(rdp->lazy_len);
>  	}
>  
> +	mutex_unlock(&rcu_state.barrier_mutex);
> +
>  	return count ? count : SHRINK_EMPTY;
>  }
>  
> @@ -1336,6 +1344,8 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
>  	unsigned long flags;
>  	unsigned long count = 0;
>  
> +	if (WARN_ON_ONCE(!cpumask_available(rcu_nocb_mask)))
> +		return 0;
>  	/*
>  	 * Protect against concurrent (de-)offloading. Otherwise nocb locking
>  	 * may be ignored or imbalanced.
> @@ -1343,7 +1353,7 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
>  	mutex_lock(&rcu_state.barrier_mutex);
>  
>  	/* Snapshot count of all CPUs */
> -	for_each_possible_cpu(cpu) {
> +	for_each_cpu(cpu, rcu_nocb_mask) {
>  		struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
>  		int _count;
>  
> -- 
> 2.34.1
>