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From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
	kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com,
	akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
	josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org,
	dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com,
	oleg@redhat.com, joel@joelfernandes.org,
	Juri Lelli <juri.lelli@redhat.com>
Subject: Re: [PATCH rcu 11/14] rcu: Make rcu_normal_after_boot writable again
Date: Thu, 16 Sep 2021 09:30:17 +0200	[thread overview]
Message-ID: <YULyiT+RbAgHxO7u@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20210915233343.3906738-11-paulmck@kernel.org>

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

  reply	other threads:[~2021-09-16  7:33 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-15 23:33 [PATCH rcu 0/14] Miscellaneous fixes for v5.16 Paul E. McKenney
2021-09-15 23:33 ` [PATCH rcu 01/14] rcu: Mark accesses to rcu_state.n_force_qs Paul E. McKenney
2021-09-15 23:33 ` [PATCH rcu 02/14] rcu-nocb: Fix a couple of tree_nocb code-style nits Paul E. McKenney
2021-09-15 23:33 ` [PATCH rcu 03/14] rcu: Eliminate rcu_implicit_dynticks_qs() local variable rnhqp Paul E. McKenney
2021-09-15 23:33 ` [PATCH rcu 04/14] rcu: Eliminate rcu_implicit_dynticks_qs() local variable ruqp Paul E. McKenney
2021-09-15 23:33 ` [PATCH rcu 05/14] doc: Add another stall-warning root cause in stallwarn.rst Paul E. McKenney
2021-09-15 23:33 ` [PATCH rcu 06/14] rcu: Fix undefined Kconfig macros Paul E. McKenney
2021-09-15 23:33 ` [PATCH rcu 07/14] rcu: Comment rcu_gp_init() code waiting for CPU-hotplug operations Paul E. McKenney
2021-09-15 23:33 ` [PATCH rcu 08/14] rcu: Move rcu_dynticks_eqs_online() to rcu_cpu_starting() Paul E. McKenney
2021-09-15 23:33 ` [PATCH rcu 09/14] rcu: Simplify rcu_report_dead() call to rcu_report_exp_rdp() Paul E. McKenney
2021-09-15 23:33 ` [PATCH rcu 10/14] rcu: Make rcutree_dying_cpu() use its "cpu" parameter Paul E. McKenney
2021-09-16  7:28   ` Peter Zijlstra
2021-09-15 23:33 ` [PATCH rcu 11/14] rcu: Make rcu_normal_after_boot writable again Paul E. McKenney
2021-09-16  7:30   ` Peter Zijlstra [this message]
2021-09-16 13:57     ` Paul E. McKenney
2021-09-16 14:27       ` Peter Zijlstra
2021-09-16 16:53         ` Paul E. McKenney
2021-09-15 23:33 ` [PATCH rcu 12/14] rcu: Make rcu update module parameters world-readable Paul E. McKenney
2021-09-15 23:33 ` [PATCH rcu 13/14] rcu: Fix existing exp request check in sync_sched_exp_online_cleanup() Paul E. McKenney
2021-09-15 23:33 ` [PATCH rcu 14/14] rcu: Avoid unneeded function call in rcu_read_unlock() Paul E. McKenney

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