Date:   Wed, 19 Apr 2023 15:36:25 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Victor Hassan <victor@...winnertech.com>, fweisbec@...il.com,
        mingo@...nel.org, jindong.yue@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] tick/broadcast: Do not set oneshot_mask except
 was_periodic was true

Le Sat, Apr 15, 2023 at 11:01:51PM +0200, Thomas Gleixner a écrit :
> @@ -1020,48 +1021,89 @@ static inline ktime_t tick_get_next_peri
>  /**
>   * tick_broadcast_setup_oneshot - setup the broadcast device
>   */
> -static void tick_broadcast_setup_oneshot(struct clock_event_device *bc)
> +static void tick_broadcast_setup_oneshot(struct clock_event_device *bc,
> +					 bool from_periodic)
>  {
>  	int cpu = smp_processor_id();
> +	ktime_t nexttick = 0;
>  
>  	if (!bc)
>  		return;
>  
>  	/* Set it up only once ! */
> -	if (bc->event_handler != tick_handle_oneshot_broadcast) {
> -		int was_periodic = clockevent_state_periodic(bc);
> -
> -		bc->event_handler = tick_handle_oneshot_broadcast;
> -
> +	if (bc->event_handler == tick_handle_oneshot_broadcast) {
>  		/*
> -		 * We must be careful here. There might be other CPUs
> -		 * waiting for periodic broadcast. We need to set the
> -		 * oneshot_mask bits for those and program the
> -		 * broadcast device to fire.
> +		 * The CPU which switches from periodic to oneshot mode
> +		 * sets the broadcast oneshot bit for all other CPUs which
> +		 * are in the general (periodic) broadcast mask to ensure
> +		 * that CPUs which wait for the periodic broadcast are
> +		 * woken up.
> +		 *
> +		 * Clear the bit for the local CPU as the set bit would
> +		 * prevent the first tick_broadcast_enter() after this CPU
> +		 * switched to oneshot state to program the broadcast
> +		 * device.
>  		 */
> +		tick_broadcast_clear_oneshot(cpu);

So this path is reached when we setup/exchange a new tick device
on a CPU after the broadcast device has been set to oneshot, right?

Why does it have a specific treatment? Is it for optimization? Or am I
missing a correctness based reason?

> +	}
> +
> +
> +	bc->event_handler = tick_handle_oneshot_broadcast;
> +	bc->next_event = KTIME_MAX;
> +
> +	/*
> +	 * When the tick mode is switched from periodic to oneshot it must
> +	 * be ensured that CPUs which are waiting for periodic broadcast
> +	 * get their wake-up at the next tick.  This is achieved by ORing
> +	 * tick_broadcast_mask into tick_broadcast_oneshot_mask.
> +	 *
> +	 * For other callers, e.g. broadcast device replacement,
> +	 * tick_broadcast_oneshot_mask must not be touched as this would
> +	 * set bits for CPUs which are already NOHZ, but not idle. Their
> +	 * next tick_broadcast_enter() would observe the bit set and fail
> +	 * to update the expiry time and the broadcast event device.
> +	 */
> +	if (from_periodic) {
>  		cpumask_copy(tmpmask, tick_broadcast_mask);
> +		/* Remove the local CPU as it is obviously not idle */
>  		cpumask_clear_cpu(cpu, tmpmask);
> -		cpumask_or(tick_broadcast_oneshot_mask,
> -			   tick_broadcast_oneshot_mask, tmpmask);
> +		cpumask_or(tick_broadcast_oneshot_mask, tick_broadcast_oneshot_mask, tmpmask);
>  
> -		if (was_periodic && !cpumask_empty(tmpmask)) {
> -			ktime_t nextevt = tick_get_next_period();
> +		/*
> +		 * Ensure that the oneshot broadcast handler will wake the
> +		 * CPUs which are still waiting for periodic broadcast.
> +		 */
> +		nexttick = tick_get_next_period();
> +		tick_broadcast_init_next_event(tmpmask, nexttick);
>  
> -			clockevents_switch_state(bc, CLOCK_EVT_STATE_ONESHOT);
> -			tick_broadcast_init_next_event(tmpmask, nextevt);
> -			tick_broadcast_set_event(bc, cpu, nextevt);
> -		} else
> -			bc->next_event = KTIME_MAX;
> -	} else {
>  		/*
> -		 * The first cpu which switches to oneshot mode sets
> -		 * the bit for all other cpus which are in the general
> -		 * (periodic) broadcast mask. So the bit is set and
> -		 * would prevent the first broadcast enter after this
> -		 * to program the bc device.
> +		 * If the underlying broadcast clock event device is
> +		 * already in oneshot state, then there is nothing to do.
> +		 * The device was already armed for the next tick
> +		 * in tick_handle_broadcast_periodic()
>  		 */
> -		tick_broadcast_clear_oneshot(cpu);
> +		if (clockevent_state_oneshot(bc))
> +			return;
>  	}
> +
> +	/*
> +	 * When switching from periodic to oneshot mode arm the broadcast
> +	 * device for the next tick.
> +	 *
> +	 * If the broadcast device has been replaced in oneshot mode and
> +	 * the oneshot broadcast mask is not empty, then arm it to expire
> +	 * immediately in order to reevaluate the next expiring timer.
> +	 * nexttick is 0 and therefore in the past which will cause the
> +	 * clockevent code to force an event.
> +	 *
> +	 * For both cases the programming can be avoided when the oneshot
> +	 * broadcast mask is empty.
> +	 *
> +	 * tick_broadcast_set_event() implicitly switches the broadcast
> +	 * device to oneshot state.
> +	 */
> +	if (!cpumask_empty(tick_broadcast_oneshot_mask))
> +		tick_broadcast_set_event(bc, cpu, nexttick);

For the case where the other CPUs have already installed their
tick devices and if that function is called with from_periodic=true,
the other CPUs will notice the oneshot change on their next call to
tick_broadcast_enter() thanks to the lock, right? So the tick broadcast
will keep firing until all CPUs have been through idle once and called
tick_broadcast_exit(), right? Because only them can clear themselves
from tick_broadcast_oneshot_mask, am I understanding this correctly?

I'm trying to find the opportunity for a race with dev->next_event
being seen as too far ahead in the future but can't manage so far...

Thanks.

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