Re: [PATCH rcu 11/14] rcu: Make rcu_normal_after_boot writable again

[Peter Zijlstra]

On Thu, Sep 16, 2021 at 06:57:12AM -0700, Paul E. McKenney wrote:
> RT systems they avoid expedited grace periods by booting with either
> the rcupdate.rcu_normal or the rcupdate.rcu_normal_after_boot kernel
> boot parameters.  And here is the definition for the latter:
> 
> static int rcu_normal_after_boot = IS_ENABLED(CONFIG_PREEMPT_RT);
> 
> In other words, RT systems shut off expedited grace periods by default,
> and are thus free to use nohz_full CPU or not, as they choose.  When using
> nohz_full, they can also enable expedited grace periods by booting with
> rcupdate.rcu_normal_after_boot=0.  Or not, sysadm's choice.
> 
> So I am not seeing a problem here.  What am I missing?

That wasn't at all clear to me from the Changelog. I thought it was
enabling expedited crud for RT.