Re: [patch 1/3] tick: Remove pointless cpu valid check in hotplug code

[Frederic Weisbecker <frederic@kernel.org>]

On Sun, Dec 06, 2020 at 10:12:54PM +0100, Thomas Gleixner wrote:
> tick_handover_do_timer() which is invoked when a CPU is unplugged has a
> check for cpumask_first(cpu_online_mask) when it tries to hand over the
> tick update duty.
> 
> Checking the result of cpumask_first() there is pointless because if the
> online mask is empty at this point, then this would be the last CPU in the
> system going offline, which is impossible. There is always at least one CPU
> remaining. If online mask would be really empty then the timer duty would
> be the least of the resulting problems.
> 
> Remove the well meant check simply because it is pointless and confusing.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
>  kernel/time/tick-common.c |   10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
> 
> --- a/kernel/time/tick-common.c
> +++ b/kernel/time/tick-common.c
> @@ -407,17 +407,13 @@ EXPORT_SYMBOL_GPL(tick_broadcast_oneshot
>  /*
>   * Transfer the do_timer job away from a dying cpu.
>   *
> - * Called with interrupts disabled. Not locking required. If
> + * Called with interrupts disabled. No locking required. If
>   * tick_do_timer_cpu is owned by this cpu, nothing can change it.
>   */
>  void tick_handover_do_timer(void)
>  {
> -	if (tick_do_timer_cpu == smp_processor_id()) {
> -		int cpu = cpumask_first(cpu_online_mask);
> -
> -		tick_do_timer_cpu = (cpu < nr_cpu_ids) ? cpu :
> -			TICK_DO_TIMER_NONE;
> -	}
> +	if (tick_do_timer_cpu == smp_processor_id())
> +		tick_do_timer_cpu = cpumask_first(cpu_online_mask);
>  }

BTW since we have that, why do we need:

static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
{
	/*
	 * If this CPU is offline and it is the one which updates
	 * jiffies, then give up the assignment and let it be taken by
	 * the CPU which runs the tick timer next. If we don't drop
	 * this here the jiffies might be stale and do_timer() never
	 * invoked.
	 */
	if (unlikely(!cpu_online(cpu))) {
		if (cpu == tick_do_timer_cpu)
			tick_do_timer_cpu = TICK_DO_TIMER_NONE;


We should only enter idle with an offline CPU after calling
tick_handover_do_timer() so (cpu == tick_do_timer_cpu) shouldn't be possible.

Or am I missing something?