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Date: Fri, 11 Dec 2020 23:21:04 +0100 From: Frederic Weisbecker <frederic@...nel.org> To: Thomas Gleixner <tglx@...utronix.de> Cc: LKML <linux-kernel@...r.kernel.org>, Marco Elver <elver@...gle.com>, kasan-dev <kasan-dev@...glegroups.com>, Peter Zijlstra <peterz@...radead.org>, "Paul E. McKenney" <paulmck@...nel.org>, Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>, Naresh Kamboju <naresh.kamboju@...aro.org> Subject: Re: [patch 1/3] tick: Remove pointless cpu valid check in hotplug code 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@...utronix.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 <frederic@...nel.org> Thanks.
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