Re: [PATCH v3 1/2] rcu: Enable rcu_normal_wake_from_gp on small systems

[Uladzislau Rezki <urezki@gmail.com>]

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 <joelagnelf@nvidia.com>
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@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