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

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Message-ID: <YULyiT+RbAgHxO7u@hirez.programming.kicks-ass.net>
Date:   Thu, 16 Sep 2021 09:30:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...nel.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 Wed, Sep 15, 2021 at 04:33:40PM -0700, Paul E. McKenney wrote:
> From: Juri Lelli <juri.lelli@...hat.com>
> 
> 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 (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.
> 
> Reflect the change in synchronize_rcu_expedited_wait() by removing the
> WARN related to CONFIG_PREEMPT_RT.
> 
> Signed-off-by: Juri Lelli <juri.lelli@...hat.com>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> ---
>  kernel/rcu/tree_exp.h | 1 -
>  kernel/rcu/update.c   | 2 +-
>  2 files changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index 2796084ef85a..d9e4f8eb9ae2 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -512,7 +512,6 @@ static void synchronize_rcu_expedited_wait(void)
>  		j = READ_ONCE(jiffies_till_first_fqs);
>  		if (synchronize_rcu_expedited_wait_once(j + HZ))
>  			return;
> -		WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_RT));
>  	}
>  
>  	for (;;) {

Urgh... I really don't like this. That would force RT applications to
always use NOHZ_FULL which isn't always possible or desired.

WTH does that netns crud need expedited so much?