linux-kernel - Re: [PATCH rcu 11/14] rcu: Make rcu_normal_after

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Message-ID: <YUNUQ564PKq9wtTF@hirez.programming.kicks-ass.net>
Date:   Thu, 16 Sep 2021 16:27:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org,
        Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [PATCH rcu 11/14] rcu: Make rcu_normal_after_boot writable again

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.