linux-kernel - Re: [PATCH] time/tick-broadcast: Fix tick_broadcast

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Date:   Tue, 25 Jun 2019 01:12:23 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, mingo@...hat.com, tglx@...utronix.de
Subject: Re: [PATCH] time/tick-broadcast: Fix tick_broadcast_offline()
 lockdep complaint

On Fri, Jun 21, 2019 at 04:46:02PM -0700, Paul E. McKenney wrote:
> @@ -3097,13 +3126,21 @@ static void sched_tick_remote(struct work_struct *work)
>  	/*
>  	 * Run the remote tick once per second (1Hz). This arbitrary
>  	 * frequency is large enough to avoid overload but short enough
> -	 * to keep scheduler internal stats reasonably up to date.
> +	 * to keep scheduler internal stats reasonably up to date.  But
> +	 * first update state to reflect hotplug activity if required.
>  	 */
> +	os = atomic_read(&twork->state);
> +	if (os) {
> +		WARN_ON_ONCE(os != TICK_SCHED_REMOTE_OFFLINING);
> +		if (atomic_inc_not_zero(&twork->state))
> +			return;

Using inc makes me a bit nervous here. If we do so, we should somewhow
make sure that we never exceed a value higher than TICK_SCHED_REMOTE_OFFLINE
by accident.

atomic_xchg() is probably a bit costlier but also safer as it allows
us to check both the old and the new value. That path shouldn't be critically fast
after all.

> +	}
>  	queue_delayed_work(system_unbound_wq, dwork, HZ);
>  }
>  
>  static void sched_tick_start(int cpu)
>  {
> +	int os;
>  	struct tick_work *twork;
>  
>  	if (housekeeping_cpu(cpu, HK_FLAG_TICK))
> @@ -3112,15 +3149,20 @@ static void sched_tick_start(int cpu)
>  	WARN_ON_ONCE(!tick_work_cpu);
>  
>  	twork = per_cpu_ptr(tick_work_cpu, cpu);
> -	twork->cpu = cpu;
> -	INIT_DELAYED_WORK(&twork->work, sched_tick_remote);
> -	queue_delayed_work(system_unbound_wq, &twork->work, HZ);
> +	os = atomic_xchg(&twork->state, TICK_SCHED_REMOTE_RUNNING);
> +	WARN_ON_ONCE(os == TICK_SCHED_REMOTE_RUNNING);

See if we use atomic_inc(), we would need to also WARN(os > TICK_SCHED_REMOTE_OFFLINE).

> +	if (os == TICK_SCHED_REMOTE_OFFLINE) {
> +		twork->cpu = cpu;
> +		INIT_DELAYED_WORK(&twork->work, sched_tick_remote);
> +		queue_delayed_work(system_unbound_wq, &twork->work, HZ);
> +	}
>  }

Thanks.