Messages in this thread | | | Date | Fri, 11 Dec 2020 23:31:55 +0100 | From | Frederic Weisbecker <> | Subject | Re: [patch 1/3] tick: Remove pointless cpu valid check in hotplug code |
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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?
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