Re: [PATCH v3] watchdog: add watchdog_cpumask sysctl to assist nohz

[Chris Metcalf <cmetcalf@ezchip.com>]

On 04/02/2015 02:06 PM, Peter Zijlstra wrote:
> On Thu, Apr 02, 2015 at 01:39:28PM -0400, cmetcalf@ezchip.com wrote:
>> @@ -431,6 +434,10 @@ static void watchdog_enable(unsigned int cpu)
>>   	hrtimer_init(hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
>>   	hrtimer->function = watchdog_timer_fn;
>>   
>> +	/* Exit if the cpu is not allowed for watchdog. */
>> +	if (!cpumask_test_cpu(cpu, watchdog_mask))
>> +		do_exit(0);
>> +
> Ick, that doesn't look right for smpboot threads.

I didn't see a better way to make this happen without adding
a bunch of infrastructure to the smpboot thread mechanism
to use a cpumask other than for_each_online_cpu().  The exit
seems benign in my testing, but I agree it's not the cleanest
way to express what we're trying to do here.

Perhaps something like an optional cpumask_t pointer in
struct smp_hotplug_thread, which if present specifies the
cpus to run on, and otherwise we stick with cpu_online_mask?

-- 
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com