rcu: Enable rcu_normal_wake_from_gp on small systems

rcu: Enable rcu_normal_wake_from_gp on small systems

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@nvidia.com>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.upadhyay@kernel.org>