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

[Paul E. McKenney]

On Wed, Mar 29, 2023 at 06:02:03PM +0200, 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.
> 
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>

Looks plausible.  ;-)

What are you doing to test this?  For that matter, what should rcutorture
be doing to test this?  My guess is that the current callback flooding in
rcu_torture_fwd_prog_cr() should do the trick, but figured I should ask.

							Thanx, Paul

> ---
>  kernel/rcu/tree_nocb.h | 17 ++++++++++++++---
>  1 file changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index dfa9c10d6727..43229d2b0c44 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1319,13 +1319,22 @@ 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. */
> +	if (!mutex_trylock(&rcu_state.barrier_mutex))
> +		return 0;
> +
>  	/* 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 +1345,8 @@ 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.
> @@ -1351,11 +1362,11 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
>  	}
>  
>  	/* 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;
>  
> -		if (!rcu_rdp_is_offloaded(rdp))
> +		if (WARN_ON_ONCE(!rcu_rdp_is_offloaded(rdp)))
>  			continue;
>  
>  		if (!READ_ONCE(rdp->lazy_len))
> -- 
> 2.34.1
>