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

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Message-ID: <20210916165324.GG4156@paulmck-ThinkPad-P17-Gen-1>
Date:   Thu, 16 Sep 2021 09:53:24 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.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 04:27:15PM +0200, Peter Zijlstra wrote:
> 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.

This paragraph of the current commit log covers that point:

	Make rcupdate.rcu_normal_after_boot= again writeable on RT (if
	NO_HZ_ FULL is defined), but keep its default value to 1 (enabled)
	to avoid regressions. Users who need synchronize_rcu_expedited()
	will boot with rcupdate.rcu_normal_after_ boot=0 in the kernel
	cmdline.

Does that cover it from your viewpoint?  If not, any suggested changes
for clarification?

							Thanx, Paul