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SubjectRe: [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@intel.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@linutronix.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@intel.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@linutronix.de>
Signed-off-by: Zqiang <qiang1.zhang@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.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__);
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