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[95.203.1.180]) by smtp.gmail.com with ESMTPSA id 2adb3069b0e04-5536773197asm1879585e87.213.2025.06.11.02.25.25 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 11 Jun 2025 02:25:26 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Wed, 11 Jun 2025 11:25:24 +0200 To: Joel Fernandes Cc: "Uladzislau Rezki (Sony)" , "Paul E . McKenney" , Joel Fernandes , Neeraj upadhyay , RCU , LKML , Frederic Weisbecker , Oleksiy Avramchenko Subject: Re: [PATCH 2/3] rcu: Enable rcu_normal_wake_from_gp on small systems Message-ID: References: <20250610173450.107293-1-urezki@gmail.com> <20250610173450.107293-2-urezki@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Tue, Jun 10, 2025 at 02:34:10PM -0400, Joel Fernandes wrote: > > > 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): > > > > > > 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 > > ... > > > > > > > > 0 - 9999 : 1 > > 10000 - 19999 : 234 > > 20000 - 29999 : 6678 > > 30000 - 39999 : 33463 > > 40000 - 49999 : 20669 > > 50000 - 59999 : 2766 > > 60000 - 69999 : 183 > > ... > > > > > > Signed-off-by: Uladzislau Rezki (Sony) > > --- > > 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. > Right. That case should be fixed. -- Uladzislau Rezki