RE: [PATCH] rcu: per-cpu rcuc kthread are created only when rcutree.use_softirq=0

[Zhang, Qiang1]

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@xxxxxxxxx>
> > 
> > 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@xxxxxxxxxxxxx>

Thanks Sebastian review.

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

Hi Paul , thank you for your modification, your description is clearer.

Thanks,
Zqiang

>
>							Thanx, Paul
>
>------------------------------------------------------------------------
>
>commit 6e68b781388cfaca95a07493a060c4a6e4ee5d0f
>Author: Zqiang <qiang1.zhang@xxxxxxxxx>
>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@xxxxxxxxxxxxx>
>    Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
>    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
>
>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__);