According to this link https://lwn.net/Articles/777214/
"rcupdate.rcu_normal_after_boot= causes expedited grace-period primitives to act like their normal counterparts once init has spawned. Real-time systems desiring fast boot but wishing to avoid run-time IPIs from expedited grace periods would therefore set both rcupdate.rcu_expedited= and rcupdate.rcu_normal_after_boot=."
force expedited grace periods only at boot period to benefit from fast boot up but after that... keeping force expedited grace for normal usage just brings power consumption up and push cpu to use higher frequencies hence we use rcu_normal_after_boot which will switch to rcu_normal after boot up is completed and enjoy the power efficiency the it brings from rcu_normal
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