linux-kernel - Re: [PATCH 1/3] [lockup detector] sync touch_*

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Date:	Wed, 1 Sep 2010 11:42:10 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Don Zickus <dzickus@...hat.com>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"fweisbec@...il.com" <fweisbec@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] [lockup detector] sync touch_*_watchdog back to old semantics

On Wednesday, September 1, 2010, Ingo Molnar <mingo@...e.hu> wrote:
>
> * Cyrill Gorcunov <gorcunov@...il.com> wrote:
>
>> On 9/1/10, Cyrill Gorcunov <gorcunov@...il.com> wrote:
>> > On 9/1/10, Ingo Molnar <mingo@...e.hu> wrote:
>> >>
>> >> * Don Zickus <dzickus@...hat.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...
>
> Ok - can you cite the old watchdog code, did it really do a nr_cpus
> loop?
>
> Thanks,
>
>         Ingo
>

Yes, previous touch_nmi_watchdog really did a loop as
for_each_present_cpu and touching per-cpu variable

(bad format)

void touch_nmi_watchdog(void)
{
if (nmi_watchdog_active()) {
  unsigned cpu;
  for_each_present_cpu(cpu) {
    if (per_cpu(nmi_touch,cpu) != 1)
       per_cpu(nmi_touch, cpu) = 1;
    }
  }
touch_softlockup_watchdog();
}
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