Message-ID: <c88b6da1-1c23-8bd6-7826-5976032246a9@allwinnertech.com>
Date:   Sat, 6 May 2023 20:09:15 +0800
From:   Victor Hassan <victor@...winnertech.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Frederic Weisbecker <frederic@...nel.org>
Cc:     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



On 5/2/2023 8:38 PM, Thomas Gleixner wrote:
> On Tue, May 02 2023 at 13:19, Frederic Weisbecker wrote:
>> On Fri, Apr 21, 2023 at 11:32:15PM +0200, Thomas Gleixner wrote:
>> Ok I get the check_clock game. But then, why do we need to reprogram
>> again the broadcast device to fire in one jiffy if the caller is
>> tick_nohz_switch_to_nohz() (that is the (bc->event_handler ==
>> tick_handle_oneshot_broadcast) branch)? In that case the broadcast device
>> should have been programmed already by the CPU that first switched the
>> current broadcast device, right?
> 
> That clearly lacks a return in that path.
> 
>>> It seems I failed miserably to explain that coherently with the tons of
>>> comments added. Hrmpf :(
>>
>> Don't pay too much attention, confusion is my vehicle to explore any code
>> that I'm not used to. But yes I must confess the
>> (bc->event_handler == tick_handle_oneshot_broadcast) may deserve a comment
>> remaining where we come from (ie: low-res hrtimer softirq).
> 
> Updated patch below.
> 
> Thanks,
> 
>          tglx
> ---
> --- a/kernel/time/tick-broadcast.c
> +++ b/kernel/time/tick-broadcast.c
> @@ -35,14 +35,15 @@ static __cacheline_aligned_in_smp DEFINE
>   #ifdef CONFIG_TICK_ONESHOT
>   static DEFINE_PER_CPU(struct clock_event_device *, tick_oneshot_wakeup_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);
>   static void tick_broadcast_clear_oneshot(int cpu);
>   static void tick_resume_broadcast_oneshot(struct clock_event_device *bc);
>   # ifdef CONFIG_HOTPLUG_CPU
>   static void tick_broadcast_oneshot_offline(unsigned int cpu);
>   # endif
>   #else
> -static inline void tick_broadcast_setup_oneshot(struct clock_event_device *bc) { BUG(); }
> +static inline void
> +tick_broadcast_setup_oneshot(struct clock_event_device *bc, bool from_periodic) { BUG(); }
>   static inline void tick_broadcast_clear_oneshot(int cpu) { }
>   static inline void tick_resume_broadcast_oneshot(struct clock_event_device *bc) { }
>   # ifdef CONFIG_HOTPLUG_CPU
> @@ -264,7 +265,7 @@ int tick_device_uses_broadcast(struct cl
>   		if (tick_broadcast_device.mode == TICKDEV_MODE_PERIODIC)
>   			tick_broadcast_start_periodic(bc);
>   		else
> -			tick_broadcast_setup_oneshot(bc);
> +			tick_broadcast_setup_oneshot(bc, false);
>   		ret = 1;
>   	} else {
>   		/*
> @@ -500,7 +501,7 @@ void tick_broadcast_control(enum tick_br
>   			if (tick_broadcast_device.mode == TICKDEV_MODE_PERIODIC)
>   				tick_broadcast_start_periodic(bc);
>   			else
> -				tick_broadcast_setup_oneshot(bc);
> +				tick_broadcast_setup_oneshot(bc, false);
>   		}
>   	}
>   out:
> @@ -1020,48 +1021,95 @@ 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;
> -
> +	/*
> +	 * When the broadcast device was switched to oneshot by the first
> +	 * CPU handling the NOHZ change, the other CPUs will reach this
> +	 * code via hrtimer_run_queues() -> tick_check_oneshot_change()
> +	 * too. Set up the broadcast device only once!
> +	 */
> +	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 switched from periodic to oneshot mode
> +		 * set 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);
> +		return;
> +	}
> +
> +
> +	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);
>   }
>   
>   /*
> @@ -1070,14 +1118,16 @@ static void tick_broadcast_setup_oneshot
>   void tick_broadcast_switch_to_oneshot(void)
>   {
>   	struct clock_event_device *bc;
> +	enum tick_device_mode oldmode;
>   	unsigned long flags;
>   
>   	raw_spin_lock_irqsave(&tick_broadcast_lock, flags);
>   
> +	oldmode = tick_broadcast_device.mode;
>   	tick_broadcast_device.mode = TICKDEV_MODE_ONESHOT;
>   	bc = tick_broadcast_device.evtdev;
>   	if (bc)
> -		tick_broadcast_setup_oneshot(bc);
> +		tick_broadcast_setup_oneshot(bc, oldmode == TICKDEV_MODE_PERIODIC);
>   
>   	raw_spin_unlock_irqrestore(&tick_broadcast_lock, flags);
>   }

That looks good to me.

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