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Date:   Fri, 20 Jan 2023 23:25:40 +0100
From:   Frederic Weisbecker <frederic@kernel.org>
To:     "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc:     linux-kernel@vger.kernel.org,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Zhouyi Zhou <zhouzhouyi@gmail.com>,
        Will Deacon <will@kernel.org>, Marc Zyngier <maz@kernel.org>,
        rcu <rcu@vger.kernel.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] tick/nohz: Fix cpu_is_hotpluggable() by checking with
 nohz subsystem
Message-ID: <Y8sU5ADeKpow5aUY@lothringen>
References: <20230119204434.4017605-1-joel@joelfernandes.org>
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In-Reply-To: <20230119204434.4017605-1-joel@joelfernandes.org>
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On Thu, Jan 19, 2023 at 08:44:35PM +0000, Joel Fernandes (Google) wrote:
> -static int tick_nohz_cpu_down(unsigned int cpu)
> +static int tick_nohz_cpu_hotplug_ret(unsigned int cpu)
>  {
>  	/*
>  	 * The tick_do_timer_cpu CPU handles housekeeping duty (unbound
> @@ -522,6 +522,16 @@ static int tick_nohz_cpu_down(unsigned int cpu)
>  	return 0;
>  }
>  
> +static int tick_nohz_cpu_down(unsigned int cpu)
> +{
> +	return tick_nohz_cpu_hotplug_ret(cpu);
> +}
> +
> +bool tick_nohz_cpu_hotpluggable(unsigned int cpu)
> +{
> +	return tick_nohz_cpu_hotplug_ret(cpu) == 0;

This is still calling the hotplug function for the CPU in order to know if the
CPU is hotpluggable...

Why not:

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index ba2ac1469d47..a46506f7ec6d 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -532,7 +532,7 @@ 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
@@ -540,8 +540,13 @@ 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)


>  void __init tick_nohz_init(void)
>  {
>  	int cpu, ret;
> -- 
> 2.39.0.246.g2a6d74b583-goog
>