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 <joelagn...@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) <ure...@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
