Re: [PATCH 1/3] [lockup detector] sync touch_*_watchdog back to old semantics

Cyrill Gorcunov
2010-09-01T07:01:11Z

On 9/1/10, Cyrill Gorcunov <gorcunov@gmail.com> wrote:
> On 9/1/10, Ingo Molnar <mingo@elte.hu> wrote:
>>
>> * Don Zickus <dzickus@redhat.com> wrote:
>>
>>> void touch_nmi_watchdog(void)
>>> {
>>> - __get_cpu_var(watchdog_nmi_touch) = true;
>>> + if (watchdog_enabled) {
>>> + unsigned cpu;
>>> +
>>> + for_each_present_cpu(cpu) {
>>> + if (per_cpu(watchdog_nmi_touch, cpu) != true)
>>> + per_cpu(watchdog_nmi_touch, cpu) = true;
>>> + }
>>
>> Hm, this is going to be a scalability nightmare with lots of CPUs. Not
>> only do we have a nr_cpus loop, but we touch per-cpu areas of _other_
>> CPUs - a big scalability nono.
>>
>> Why do we need to do this? We never needed to touch other CPU's NMI
>> lockup accounting data areas - why has this changed? The changelog does
>> not explain this.
>>
>> Thanks,
>>
>> Ingo
>>
> I believe this came from old nmi watchdog code where it might be
> useful when nmi watchdog activated via io-apic. I'm trying to figure
> out if we really need it still.
>
Well, we can't drop it or make per-cpu specific, for example we need
it in case of panic with watchdog enabled and panic timeout set, or
boot delay set and etc. Seems same applies to printk_delay. Hmm...