linux-kernel - Re: [PATCH 2/3] rcu: Defer RCU kthreads wakeup when CPU is dying

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Date: Wed, 20 Dec 2023 16:13:24 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: Z qiang <qiang.zhang1211@...il.com>
Cc: LKML <linux-kernel@...r.kernel.org>, Boqun Feng <boqun.feng@...il.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Neeraj Upadhyay <neeraj.upadhyay@....com>,
	Uladzislau Rezki <urezki@...il.com>, rcu <rcu@...r.kernel.org>,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 2/3] rcu: Defer RCU kthreads wakeup when CPU is dying

Le Wed, Dec 20, 2023 at 04:24:35PM +0800, Z qiang a écrit :
> >
> > When the CPU goes idle for the last time during the CPU down hotplug
> > process, RCU reports a final quiescent state for the current CPU. If
> > this quiescent state propagates up to the top, some tasks may then be
> > woken up to complete the grace period: the main grace period kthread
> > and/or the expedited main workqueue (or kworker).
> >
> > If those kthreads have a SCHED_FIFO policy, the wake up can indirectly
> > arm the RT bandwith timer to the local offline CPU. Since this happens
> > after hrtimers have been migrated at CPUHP_AP_HRTIMERS_DYING stage, the
> > timer gets ignored. Therefore if the RCU kthreads are waiting for RT
> > bandwidth to be available, they may never be actually scheduled.
> >
> 
> In the rcutree_report_cpu_dead(), the rcuog kthreads may also be wakeup in
> do_nocb_deferred_wakeup(), if the rcuog kthreads is rt-fifo and wakeup happen,
> the rt_period_active is set 1 and enqueue hrtimer to offline CPU in
> do_start_rt_bandwidth(),
> after that, we invoke swake_up_one_online() send ipi to online CPU, due to the
> rt_period_active is 1, the rt-bandwith hrtimer will not enqueue to online CPU.
> any thoughts?

Duh, you're right, that one too. How many more? This hrtimer situation is scary...

Thanks.