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

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Message-ID: <87v9d7g9pv.fsf@nanos.tec.linutronix.de>
Date:   Sat, 12 Dec 2020 01:16:12 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Frederic Weisbecker <frederic@...nel.org>
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 Fri, Dec 11 2020 at 23:21, Frederic Weisbecker wrote:
> 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
>> @@ -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?

No. Because we'd need to add the NONE magic to NOHZ=n kernels which does
not make sense.

Thanks,

        tglx