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Date:   Tue, 22 Dec 2020 02:37:08 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Fabio Estevam <festevam@...il.com>, stable@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Len Brown <lenb@...nel.org>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        NXP Linux Team <linux-imx@....com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>
Subject: [PATCH 0/4] sched/idle: Fix missing need_resched() checks after rcu_idle_enter()

With Paul, we've been thinking that the idle loop wasn't twisted enough
yet to deserve 2020.

rcutorture, after some recent parameter changes, has been complaining
about a hung task.

It appears that rcu_idle_enter() may wake up a NOCB kthread but this
happens after the last generic need_resched() check. Some cpuidle drivers
fix it by chance but many others don't.

Here is a proposed bunch of fixes. I will need to also fix the
rcu_user_enter() case, likely using irq_work, since nohz_full requires
irq work to support self IPI.

Also more generally, this raise the question of local task wake_up()
under disabled interrupts. When a wake up occurs in a preempt disabled
section, it gets handled by the outer preempt_enable() call. There is no
similar mechanism when a wake up occurs with interrupts disabled. I guess
it is assumed to be handled, at worst, in the next tick. But a local irq
work would provide instant preemption once IRQs are re-enabled. Of course
this would only make sense in CONFIG_PREEMPTION, and when the tick is
disabled...

git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
	sched/idle

HEAD: f2fa6e4a070c1535b9edc9ee097167fd2b15d235

Thanks,
	Frederic
---

Frederic Weisbecker (4):
      sched/idle: Fix missing need_resched() check after rcu_idle_enter()
      cpuidle: Fix missing need_resched() check after rcu_idle_enter()
      ARM: imx6q: Fix missing need_resched() check after rcu_idle_enter()
      ACPI: processor: Fix missing need_resched() check after rcu_idle_enter()


 arch/arm/mach-imx/cpuidle-imx6q.c |  7 ++++++-
 drivers/acpi/processor_idle.c     | 10 ++++++++--
 drivers/cpuidle/cpuidle.c         | 33 +++++++++++++++++++++++++--------
 kernel/sched/idle.c               | 18 ++++++++++++------
 4 files changed, 51 insertions(+), 17 deletions(-)

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