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Date: Wed, 9 Jul 2025 13:36:56 +0200
From: Frederic Weisbecker <frederic@kernel.org>
To: neeraj.upadhyay@kernel.org
Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, paulmck@kernel.org,
	joelagnelf@nvidia.com, boqun.feng@gmail.com, urezki@gmail.com,
	rostedt@goodmis.org, mathieu.desnoyers@efficios.com,
	jiangshanlai@gmail.com, qiang.zhang1211@gmail.com,
	neeraj.iitr10@gmail.com, neeraj.upadhyay@amd.com
Subject: Re: [PATCH rcu 3/5] rcu: Enable rcu_normal_wake_from_gp on small
 systems
Message-ID: <aG5UWDQ0L3Utm8wT@localhost.localdomain>
References: <20250709104118.15532-1-neeraj.upadhyay@kernel.org>
 <20250709104118.15532-4-neeraj.upadhyay@kernel.org>
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In-Reply-To: <20250709104118.15532-4-neeraj.upadhyay@kernel.org>

Le Wed, Jul 09, 2025 at 04:11:16PM +0530, neeraj.upadhyay@kernel.org a écrit :
> From: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
> 
> 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>
> Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@kernel.org>

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>

-- 
Frederic Weisbecker
SUSE Labs