linux-kernel - Re: [PATCH 2/3] rcu: Enable rcu_normal_wake_from

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Message-ID: <d55a7d1b-d0c0-4c04-b69e-ca6737c98224@nvidia.com>
Date: Tue, 10 Jun 2025 14:34:10 -0400
From: Joel Fernandes <joelagnelf@...dia.com>
To: "Uladzislau Rezki (Sony)" <urezki@...il.com>,
 "Paul E . McKenney" <paulmck@...nel.org>,
 Joel Fernandes <joel@...lfernandes.org>,
 Neeraj upadhyay <Neeraj.Upadhyay@....com>
Cc: RCU <rcu@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
 Frederic Weisbecker <frederic@...nel.org>,
 Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: Re: [PATCH 2/3] rcu: Enable rcu_normal_wake_from_gp on small systems



On 6/10/2025 1:34 PM, 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 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>
> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> ---
>  kernel/rcu/tree.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 14d4499c6fc3..c0e0b38a08dc 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1625,7 +1625,9 @@ static void rcu_sr_put_wait_head(struct llist_node *node)
>  	atomic_set_release(&sr_wn->inuse, 0);
>  }
>  
> -/* Disabled by default. */
> +/* Enable rcu_normal_wake_from_gp automatically on small systems. */
> +#define WAKE_FROM_GP_CPU_THRESHOLD 16
> +
>  static int rcu_normal_wake_from_gp;
>  module_param(rcu_normal_wake_from_gp, int, 0644);
>  static struct workqueue_struct *sync_wq;
> @@ -4847,6 +4849,9 @@ void __init rcu_init(void)
>  	sync_wq = alloc_workqueue("sync_wq", WQ_MEM_RECLAIM, 0);
>  	WARN_ON(!sync_wq);
>  
> +	if (num_possible_cpus() <= WAKE_FROM_GP_CPU_THRESHOLD)
> +		WRITE_ONCE(rcu_normal_wake_from_gp, 1);
> +
I think this will get weird if user explictly specifies
rcutree.rcu_normal_wake_from_gp=0 ? Then we're silently overriding the param.
Maybe, initialize it to -1, and then if it was set 0 by user, don't override it.
But otherwise, set it to 1.  Per your third patch, '1' is a default, not a
forced value.

thanks,

 - Joel