Date:   Tue, 24 Jan 2023 14:56:28 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc:     linux-kernel@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Zhouyi Zhou <zhouzhouyi@gmail.com>,
        Will Deacon <will@kernel.org>, Marc Zyngier <maz@kernel.org>,
        rcu <rcu@vger.kernel.org>, stable@vger.kernel.org,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v2 RESEND] tick/nohz: Fix cpu_is_hotpluggable() by
 checking with nohz subsystem
Message-ID: <20230124225628.GZ2948950@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
References: <20230124173126.3492345-1-joel@joelfernandes.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
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In-Reply-To: <20230124173126.3492345-1-joel@joelfernandes.org>
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On Tue, Jan 24, 2023 at 05:31:26PM +0000, Joel Fernandes (Google) wrote:
> For CONFIG_NO_HZ_FULL systems, the tick_do_timer_cpu cannot be offlined.
> However, cpu_is_hotpluggable() still returns true for those CPUs. This causes
> torture tests that do offlining to end up trying to offline this CPU causing
> test failures. Such failure happens on all architectures.
> 
> Fix it by asking the opinion of the nohz subsystem on whether the CPU can
> be hotplugged.
> 
> [ Apply Frederic Weisbecker feedback on refactoring tick_nohz_cpu_down(). ]
> 
> For drivers/base/ portion:
> Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> 
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Zhouyi Zhou <zhouzhouyi@gmail.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: rcu <rcu@vger.kernel.org>
> Cc: stable@vger.kernel.org
> Fixes: 2987557f52b9 ("driver-core/cpu: Expose hotpluggability to the rest of the kernel")
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

Queued for further review and testing, thank you both!

It might be a few hours until it becomes publicly visible, but it will
get there.

							Thanx, Paul

> ---
> Sorry, resending with CC to stable.
> 
>  drivers/base/cpu.c       |  3 ++-
>  include/linux/tick.h     |  2 ++
>  kernel/time/tick-sched.c | 11 ++++++++---
>  3 files changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
> index 55405ebf23ab..450dca235a2f 100644
> --- a/drivers/base/cpu.c
> +++ b/drivers/base/cpu.c
> @@ -487,7 +487,8 @@ static const struct attribute_group *cpu_root_attr_groups[] = {
>  bool cpu_is_hotpluggable(unsigned int cpu)
>  {
>  	struct device *dev = get_cpu_device(cpu);
> -	return dev && container_of(dev, struct cpu, dev)->hotpluggable;
> +	return dev && container_of(dev, struct cpu, dev)->hotpluggable
> +		&& tick_nohz_cpu_hotpluggable(cpu);
>  }
>  EXPORT_SYMBOL_GPL(cpu_is_hotpluggable);
>  
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index bfd571f18cfd..9459fef5b857 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -216,6 +216,7 @@ extern void tick_nohz_dep_set_signal(struct task_struct *tsk,
>  				     enum tick_dep_bits bit);
>  extern void tick_nohz_dep_clear_signal(struct signal_struct *signal,
>  				       enum tick_dep_bits bit);
> +extern bool tick_nohz_cpu_hotpluggable(unsigned int cpu);
>  
>  /*
>   * The below are tick_nohz_[set,clear]_dep() wrappers that optimize off-cases
> @@ -280,6 +281,7 @@ static inline void tick_nohz_full_add_cpus_to(struct cpumask *mask) { }
>  
>  static inline void tick_nohz_dep_set_cpu(int cpu, enum tick_dep_bits bit) { }
>  static inline void tick_nohz_dep_clear_cpu(int cpu, enum tick_dep_bits bit) { }
> +static inline bool tick_nohz_cpu_hotpluggable(unsigned int cpu) { return true; }
>  
>  static inline void tick_dep_set(enum tick_dep_bits bit) { }
>  static inline void tick_dep_clear(enum tick_dep_bits bit) { }
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 9c6f661fb436..63e3e8ebcd64 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -510,7 +510,7 @@ void __init tick_nohz_full_setup(cpumask_var_t cpumask)
>  	tick_nohz_full_running = true;
>  }
>  
> -static int tick_nohz_cpu_down(unsigned int cpu)
> +bool tick_nohz_cpu_hotpluggable(unsigned int cpu)
>  {
>  	/*
>  	 * The tick_do_timer_cpu CPU handles housekeeping duty (unbound
> @@ -518,8 +518,13 @@ static int tick_nohz_cpu_down(unsigned int cpu)
>  	 * CPUs. It must remain online when nohz full is enabled.
>  	 */
>  	if (tick_nohz_full_running && tick_do_timer_cpu == cpu)
> -		return -EBUSY;
> -	return 0;
> +		return false;
> +	return true;
> +}
> +
> +static int tick_nohz_cpu_down(unsigned int cpu)
> +{
> +	return tick_nohz_cpu_hotpluggable(cpu) ? 0 : -EBUSY;
>  }
>  
>  void __init tick_nohz_init(void)
> -- 
> 2.39.1.405.gd4c25cc71f-goog
>