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

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Date:   Fri, 6 Aug 2021 09:41:05 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     josh@...htriplett.org, linux-kernel@...r.kernel.org,
        linux-rt-users@...r.kernel.org, peterz@...radead.org,
        bigeasy@...utronix.de, 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

Hi,

Thanks for reviewing Paul.

On 05/08/21 14:08, Paul E. McKenney wrote:
> On Thu, Aug 05, 2021 at 09:03:37AM -0700, Paul E. McKenney wrote:
> > On Thu, Aug 05, 2021 at 10:01:23AM +0200, Juri Lelli wrote:
> > > Certain configurations (e.g., systems that make heavy use of netns)
> > > need to use synchronize_rcu_expedited() to service RCU grace periods
> > > even after boot.
> > > 
> > > Even though synchronize_rcu_expedited() has been traditionally
> > > considered harmful for RT for the heavy use of IPIs, it is perfectly
> > > usable under certain conditions (e.g. nohz_full).
> > > 
> > > Make rcupdate.rcu_normal_after_boot= again writeable on RT, 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.
> > > 
> > > Signed-off-by: Juri Lelli <juri.lelli@...hat.com>
> > 
> > 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?
> 
> Ah, and this will also need to be reflected in the WARN_ON_ONCE()
> in synchronize_rcu_expedited_wait() in kernel/rcu/tree_exp.h.

Indeed. Will add the change as soon as I receive indication about your
first point.

Best,
Juri