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

[Neeraj Upadhyay]

On 7/3/2025 9:05 PM, Uladzislau Rezki wrote:
> 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@xxxxxxxxxx>
>> Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
>> ---
>>  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?
> 

Yes, I have queued v2 of these for testing here:

https://git.kernel.org/pub/scm/linux/kernel/git/neeraj.upadhyay/linux-rcu.git/log/?h=dev.01.07.2025

Will include them in PR for next merge window.


- Neeraj

> --
> Uladzislau Rezlo