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Subject: Re: [PATCH tip/core/rcu 02/12] timer: Report ignored local enqueue in
 nohz mode
To:     paulmck@kernel.org, rcu@vger.kernel.org
Cc:     linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org,
        jiangshanlai@gmail.com, akpm@linux-foundation.org,
        mathieu.desnoyers@efficios.com, josh@joshtriplett.org,
        tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org,
        dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com,
        oleg@redhat.com, joel@joelfernandes.org,
        Frederic Weisbecker <frederic@kernel.org>
References: <20210304002225.GA23492@paulmck-ThinkPad-P72>
 <20210304002311.23655-2-paulmck@kernel.org>
From:   "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Organization: Intel Technology Poland Sp. z o. o., KRS 101882, ul. Slowackiego
 173, 80-298 Gdansk
Message-ID: <a1bac9f6-cc67-943d-0f88-aad2f2b46bd7@intel.com>
Date:   Thu, 4 Mar 2021 12:58:54 +0100
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On 3/4/2021 1:23 AM, paulmck@kernel.org wrote:
> From: Frederic Weisbecker <frederic@kernel.org>
>
> Enqueuing a local timer after the tick has been stopped will result in
> the timer being ignored until the next random interrupt.
>
> Perform sanity checks to report these situations.
>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar<mingo@kernel.org>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

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


> ---
>   kernel/sched/core.c | 24 +++++++++++++++++++++++-
>   1 file changed, 23 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index ca2bb62..4822371 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -674,6 +674,26 @@ int get_nohz_timer_target(void)
>   	return cpu;
>   }
>   
> +static void wake_idle_assert_possible(void)
> +{
> +#ifdef CONFIG_SCHED_DEBUG
> +	/* Timers are re-evaluated after idle IRQs */
> +	if (in_hardirq())
> +		return;
> +	/*
> +	 * Same as hardirqs, assuming they are executing
> +	 * on IRQ tail. Ksoftirqd shouldn't reach here
> +	 * as the timer base wouldn't be idle. And inline
> +	 * softirq processing after a call to local_bh_enable()
> +	 * within idle loop sound too fun to be considered here.
> +	 */
> +	if (in_serving_softirq())
> +		return;
> +
> +	WARN_ON_ONCE("Late timer enqueue may be ignored\n");
> +#endif
> +}
> +
>   /*
>    * When add_timer_on() enqueues a timer into the timer wheel of an
>    * idle CPU then this timer might expire before the next timer event
> @@ -688,8 +708,10 @@ static void wake_up_idle_cpu(int cpu)
>   {
>   	struct rq *rq = cpu_rq(cpu);
>   
> -	if (cpu == smp_processor_id())
> +	if (cpu == smp_processor_id()) {
> +		wake_idle_assert_possible();
>   		return;
> +	}
>   
>   	if (set_nr_and_not_polling(rq->idle))
>   		smp_send_reschedule(cpu);