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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 Just one nit below: > > 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 | 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)