Re: [PATCH rcu 11/14] rcu: Make rcu_normal_after_boot writable again

[Peter Zijlstra]

On Wed, Sep 15, 2021 at 04:33:40PM -0700, Paul E. McKenney wrote:
> From: Juri Lelli <juri.lelli@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> ---
>  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?