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

Joel Fernandes
2023-03-24T00:42:05Z

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
>