linux-kernel - Re: [PATCH 0/4] sched/idle: Fix missing need_resched() checks after rcu_idle

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Date:   Tue, 22 Dec 2020 17:19:51 +0100
From:   "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
To:     Frederic Weisbecker <frederic@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     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>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH 0/4] sched/idle: Fix missing need_resched() checks after
 rcu_idle_enter()

On 12/22/2020 2:37 AM, Frederic Weisbecker wrote:
> 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(-)

Please feel free to add

Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

to all patches in the series.

Thanks!