linux-kernel - Re: [PATCH] rcu: Make rcu_normal_after

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Message-ID: <20210806080455.wkhlebgt7howjcrk@linutronix.de>
Date:   Fri, 6 Aug 2021 10:04:55 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Juri Lelli <juri.lelli@...hat.com>, josh@...htriplett.org,
        linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
        peterz@...radead.org, tglx@...utronix.de, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        joel@...lfernandes.org, rcu@...r.kernel.org
Subject: Re: [PATCH] rcu: Make rcu_normal_after_boot writable on RT

On 2021-08-05 09:03:37 [-0700], Paul E. McKenney wrote:
> Makes sense to me!
> 
> But would another of the -rt people be willing to give an Acked-by?
> For example, maybe they would prefer this kernel boot parameter to be
> exposed only if (!PREEMPT_RT || NO_HZ_FULL).  Or are there !NO_HZ_FULL
> situations where rcu_normal_after_boot makes sense?

Julia crafted that "rcu_normal_after_boot = 1" for RT after we had more
and more synchronize_rcu_expedited() users popping up. I would like to
keep that part (default value) since it good to have for most users.

I don't mind removing CONFIG_PREEMPT_RT part here if there are legitimate
use cases for using "rcu_normal_after_boot = 0".
Paul suggested initially to restrict that option for PREEMPT_RT and I
would follow here Paul's guidance to either remove it or restrict it to
NO_HZ_FULL in RT's case (as suggested).

> 							Thanx, Paul

Sebastian