[PATCH v2 1/2] rcu: Enable rcu_normal_wake_from_gp on small systems

[PATCH v2 1/2] rcu: Enable rcu_normal_wake_from_gp on small systems

[Uladzislau Rezki (Sony)]

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>

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

[PATCH v2 2/2] Documentation/kernel-parameters: Update rcu_normal_wake_from_gp doc

[Uladzislau Rezki (Sony)]

Update the documentation about rcu_normal_wake_from_gp parameter.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
 Documentation/admin-guide/kernel-parameters.txt | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index f1f2c0874da9..f7e4bee2b823 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5485,7 +5485,8 @@
 			echo 1 > /sys/module/rcutree/parameters/rcu_normal_wake_from_gp
 			or pass a boot parameter "rcutree.rcu_normal_wake_from_gp=1"
 
-			Default is 0.
+			Default is 1 if num_possible_cpus() <= 16 and it is not explicitly
+			disabled by the boot parameter passing 0.
 
 	rcuscale.gp_async= [KNL]
 			Measure performance of asynchronous
-- 
2.39.5

Re: [PATCH v2 1/2] rcu: Enable rcu_normal_wake_from_gp on small systems

[Joel Fernandes]

On 6/11/2025 1:56 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(system with 64 CPUs) invoking
> synchronize_rcu() demonstrates a notable latency reduction with the
> setting enabled.

Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>

Just one nit below:

> 
> 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@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;
> +	}

nit: better to just do:

	if (rcu_normal_wake_from_gp < 0 &&
	    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)

Re: [PATCH v2 2/2] Documentation/kernel-parameters: Update rcu_normal_wake_from_gp doc

[Joel Fernandes]

On 6/11/2025 1:56 PM, Uladzislau Rezki (Sony) wrote:
> Update the documentation about rcu_normal_wake_from_gp parameter.
> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>

Thanks.


> ---
>  Documentation/admin-guide/kernel-parameters.txt | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index f1f2c0874da9..f7e4bee2b823 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -5485,7 +5485,8 @@
>  			echo 1 > /sys/module/rcutree/parameters/rcu_normal_wake_from_gp
>  			or pass a boot parameter "rcutree.rcu_normal_wake_from_gp=1"
>  
> -			Default is 0.
> +			Default is 1 if num_possible_cpus() <= 16 and it is not explicitly
> +			disabled by the boot parameter passing 0.
>  
>  	rcuscale.gp_async= [KNL]
>  			Measure performance of asynchronous

Re: [PATCH v2 1/2] rcu: Enable rcu_normal_wake_from_gp on small systems

[Uladzislau Rezki]

On Wed, Jun 11, 2025 at 05:48:57PM -0400, Joel Fernandes wrote:
> 
> 
> On 6/11/2025 1:56 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(system with 64 CPUs) invoking
> > synchronize_rcu() demonstrates a notable latency reduction with the
> > setting enabled.
> 
> Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
> 
Thank you for review!

> Just one nit below:
> 
> > 
> > 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@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;
> > +	}
> 
> nit: better to just do:
> 
> 	if (rcu_normal_wake_from_gp < 0 &&
> 	    num_possible_cpus() <= WAKE_FROM_GP_CPU_THRESHOLD)
>  		rcu_normal_wake_from_gp = 1;
> 	}
> 
Two lines anyway :)

--
Uladzislau Rezki

Re: [PATCH v2 2/2] Documentation/kernel-parameters: Update rcu_normal_wake_from_gp doc

[Uladzislau Rezki]

On Wed, Jun 11, 2025 at 05:49:14PM -0400, Joel Fernandes wrote:
> 
> 
> On 6/11/2025 1:56 PM, Uladzislau Rezki (Sony) wrote:
> > Update the documentation about rcu_normal_wake_from_gp parameter.
> > 
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> 
> Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
> 
Thanks!

--
Uladzislau Rezki