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

Cyrill Gorcunov
2010-09-01T06:00:45Z

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.