linux-kernel - Re: [PATCH] rcu: per-cpu rcuc kthread are created only when rcutree.use

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Date:   Mon, 3 Jan 2022 06:34:42 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Zqiang <qiang1.zhang@...el.com>, frederic@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: per-cpu rcuc kthread are created only when
 rcutree.use_softirq=0

On Mon, Jan 03, 2022 at 02:17:11PM +0100, Sebastian Andrzej Siewior wrote:
> On 2022-01-01 08:44:26 [-0800], Paul E. McKenney wrote:
> > On Wed, Dec 29, 2021 at 12:05:10AM +0800, Zqiang wrote:
> > > In non-RT kernel, if the RCU_BOOST is enabled, the per-cpu rcuc
> > > kthread will be created, however under the rcutree.use_softirq=1,
> > > the RCU core processing only in softirq context, the rcuc kthread
> > > doesn't do anything, so remove RCU_BOOST interference.
> > > 
> > > Signed-off-by: Zqiang <qiang1.zhang@...el.com>
> > 
> > Looks sane to me, but adding Sebastian on CC for his thoughts.
> 
> Yes, it makes sense. invoke_rcu_core_kthread() is only invoked for
> !use_softirq so it makes to create the threads based on this condition.
> 
> Acked-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

Thank you both!  As usual, I could not resist a bit of wordsmithing,
please see below.

							Thanx, Paul

------------------------------------------------------------------------

commit 6e68b781388cfaca95a07493a060c4a6e4ee5d0f
Author: Zqiang <qiang1.zhang@...el.com>
Date:   Wed Dec 29 00:05:10 2021 +0800

    rcu: Create per-cpu rcuc kthreads only when rcutree.use_softirq=0
    
    The per-CPU "rcuc" kthreads are used only by kernels booted with
    rcutree.use_softirq=0, but they are nevertheless unconditionally created
    by kernels built with CONFIG_RCU_BOOST=y.  This results in "rcuc"
    kthreads being created that are never actually used.  This commit
    therefore refrains from creating these kthreads unless the kernel
    is actually booted with rcutree.use_softirq=0.
    
    Acked-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
    Signed-off-by: Zqiang <qiang1.zhang@...el.com>
    Signed-off-by: Paul E. McKenney <paulmck@...nel.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index aeea5487632f7..5c0ffee7a43a0 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2903,7 +2903,7 @@ static int __init rcu_spawn_core_kthreads(void)
 
 	for_each_possible_cpu(cpu)
 		per_cpu(rcu_data.rcu_cpu_has_work, cpu) = 0;
-	if (!IS_ENABLED(CONFIG_RCU_BOOST) && use_softirq)
+	if (use_softirq)
 		return 0;
 	WARN_ONCE(smpboot_register_percpu_thread(&rcu_cpu_thread_spec),
 		  "%s: Could not start rcuc kthread, OOM is now expected behavior\n", __func__);