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 <t...@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);
I was about to whine that this randomly chosen CPU may be idle and leave
the timekeeping stale until I realized that stop_machine() is running at that
time. Might be worth adding a comment about that.
Also why not just setting it to TICK_DO_TIMER_NONE and be done with it? Perhaps
to avoid that all the CPUs to compete and contend on jiffies update after stop
machine?
If so:
Reviewed-by: Frederic Weisbecker <frede...@kernel.org>
Thanks.
