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

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Date:   Thu, 5 Aug 2021 09:03:37 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Juri Lelli <juri.lelli@...hat.com>
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

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?

							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
>