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

[Uladzislau Rezki]

On Mon, Jul 07, 2025 at 09:19:03AM +0530, Neeraj Upadhyay wrote:
> 
> 
> 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.
> 
Thank you! I asked because i did not see both in our common RCU tree.

--
Uladzislau Rezki