On Wed, Jul 02, 2025 at 04:59:36PM +0200, Uladzislau Rezki (Sony) wrote:
> Automatically enable the rcu_normal_wake_from_gp parameter on
> systems with a small number of CPUs. The activation threshold
> is set to 16 CPUs.
>
> This helps to reduce a latency of normal synchronize_rcu() API
> by waking up GP-waiters earlier and decoupling synchronize_rcu()
> callers from regular callback handling.
>
> A benchmark running 64 parallel jobs(system with 64 CPUs) invoking
> synchronize_rcu() demonstrates a notable latency reduction with the
> setting enabled.
>
> Latency distribution (microseconds):
>
> <default>
> 0 - 9999 : 1
> 10000 - 19999 : 4
> 20000 - 29999 : 399
> 30000 - 39999 : 3197
> 40000 - 49999 : 10428
> 50000 - 59999 : 17363
> 60000 - 69999 : 15529
> 70000 - 79999 : 9287
> 80000 - 89999 : 4249
> 90000 - 99999 : 1915
> 100000 - 109999 : 922
> 110000 - 119999 : 390
> 120000 - 129999 : 187
> ...
> <default>
>
> <rcu_normal_wake_from_gp>
> 0 - 9999 : 1
> 10000 - 19999 : 234
> 20000 - 29999 : 6678
> 30000 - 39999 : 33463
> 40000 - 49999 : 20669
> 50000 - 59999 : 2766
> 60000 - 69999 : 183
> ...
> <rcu_normal_wake_from_gp>
>
> Reviewed-by: Joel Fernandes <joelagn...@nvidia.com>
> Signed-off-by: Uladzislau Rezki (Sony) <ure...@gmail.com>
> ---
> kernel/rcu/tree.c | 14 +++++++++++---
> 1 file changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index e8a4b720d7d2..b88ceb35cebd 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1625,8 +1625,10 @@ static void rcu_sr_put_wait_head(struct llist_node
> *node)
> atomic_set_release(&sr_wn->inuse, 0);
> }
>
> -/* Disabled by default. */
> -static int rcu_normal_wake_from_gp;
> +/* Enable rcu_normal_wake_from_gp automatically on small systems. */
> +#define WAKE_FROM_GP_CPU_THRESHOLD 16
> +
> +static int rcu_normal_wake_from_gp = -1;
> module_param(rcu_normal_wake_from_gp, int, 0644);
> static struct workqueue_struct *sync_wq;
>
> @@ -3239,7 +3241,7 @@ static void synchronize_rcu_normal(void)
>
> trace_rcu_sr_normal(rcu_state.name, &rs.head, TPS("request"));
>
> - if (!READ_ONCE(rcu_normal_wake_from_gp)) {
> + if (READ_ONCE(rcu_normal_wake_from_gp) < 1) {
> wait_rcu_gp(call_rcu_hurry);
> goto trace_complete_out;
> }
> @@ -4843,6 +4845,12 @@ void __init rcu_init(void)
> sync_wq = alloc_workqueue("sync_wq", WQ_MEM_RECLAIM, 0);
> WARN_ON(!sync_wq);
>
> + /* Respect if explicitly disabled via a boot parameter. */
> + if (rcu_normal_wake_from_gp < 0) {
> + if (num_possible_cpus() <= WAKE_FROM_GP_CPU_THRESHOLD)
> + rcu_normal_wake_from_gp = 1;
> + }
> +
> /* Fill in default value for rcutree.qovld boot parameter. */
> /* -After- the rcu_node ->lock fields are initialized! */
> if (qovld < 0)
> --
> 2.39.5
>
Neeraj, are you planning to take this for next merge window?
--
Uladzislau Rezlo
