From: "Paul E. McKenney" <paulmck@kernel.org>
To: Juri Lelli <juri.lelli@redhat.com>
Cc: josh@joshtriplett.org, linux-kernel@vger.kernel.org,
linux-rt-users@vger.kernel.org, peterz@infradead.org,
bigeasy@linutronix.de, tglx@linutronix.de, rostedt@goodmis.org,
mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com,
joel@joelfernandes.org, rcu@vger.kernel.org
Subject: Re: [PATCH] rcu: Make rcu_normal_after_boot writable on RT
Date: Thu, 5 Aug 2021 14:08:24 -0700 [thread overview]
Message-ID: <20210805210824.GA1203206@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <20210805160337.GI4397@paulmck-ThinkPad-P17-Gen-1>
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@redhat.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.
Thanx, Paul
> > ---
> > kernel/rcu/update.c | 2 --
> > 1 file changed, 2 deletions(-)
> >
> > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> > index c21b38cc25e9..0fdbf937edac 100644
> > --- a/kernel/rcu/update.c
> > +++ b/kernel/rcu/update.c
> > @@ -57,9 +57,7 @@
> > module_param(rcu_expedited, int, 0);
> > module_param(rcu_normal, int, 0);
> > static int rcu_normal_after_boot = IS_ENABLED(CONFIG_PREEMPT_RT);
> > -#ifndef CONFIG_PREEMPT_RT
> > module_param(rcu_normal_after_boot, int, 0);
> > -#endif
> > #endif /* #ifndef CONFIG_TINY_RCU */
> >
> > #ifdef CONFIG_DEBUG_LOCK_ALLOC
> > --
> > 2.31.1
> >
next prev parent reply other threads:[~2021-08-05 21:08 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-05 8:01 [PATCH] rcu: Make rcu_normal_after_boot writable on RT Juri Lelli
2021-08-05 16:03 ` Paul E. McKenney
2021-08-05 21:08 ` Paul E. McKenney [this message]
2021-08-06 7:41 ` Juri Lelli
2021-08-06 8:04 ` Sebastian Andrzej Siewior
2021-08-06 17:44 ` Paul E. McKenney
2021-08-09 8:37 ` Juri Lelli
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