Re: [PATCH v8 2/3] watchdog: add watchdog_cpumask sysctl to assist nohz

[Chris Metcalf <cmetcalf@ezchip.com>]

On 04/16/2015 09:31 PM, Chai Wen wrote:
> On 04/15/2015 03:37 AM, Chris Metcalf wrote:
>> +/*
>> + * The cpumask is the mask of possible cpus that the watchdog can run
>> + * on, not the mask of cpus it is actually running on.  This allows the
>> + * user to specify a mask that will include cpus that have not yet
>> + * been brought online, if desired.
>> + */
>> +int proc_watchdog_cpumask(struct ctl_table *table, int write,
>> +			  void __user *buffer, size_t *lenp, loff_t *ppos)
>> +{
>> +	int err;
>> +
>> +	mutex_lock(&watchdog_proc_mutex);
>> +	err = proc_do_large_bitmap(table, write, buffer, lenp, ppos);
>> +	if (!err && write) {
>> +		/* Remove impossible cpus to keep sysctl output cleaner. */
>> +		cpumask_and(watchdog_cpumask, watchdog_cpumask,
>> +			    cpu_possible_mask);
>> +
>> +		if (watchdog_enabled && watchdog_thresh)
>
> If the new mask is same as the current one, then there is no need to go on ?
> cpus_equal(watchdog_cpumask, watchdog_cpumask_for_smpboot) or something else ?

It's a minor optimization, though, since the 
smpboot_update_cpumask_percpu_thread()
function will do some cpumask calls and realize that nothing has changed and
return without doing anything anyway.

In any case, with Frederic's recent suggstion, we won't have a
watchdog_cpumask_for_smpboot variable exposed anyway.

>> +			smpboot_update_cpumask_percpu_thread(&watchdog_threads,
>> +							     watchdog_cpumask);
>> +	}
>> +	mutex_unlock(&watchdog_proc_mutex);
>> +	return err;
>> +}
>> +
>>   #endif /* CONFIG_SYSCTL */
>>   
>>   void __init lockup_detector_init(void)
>>   {
>>   	set_sample_period();
>>   
>> +	/* One cpumask is allocated for smpboot to own. */
>> +	alloc_cpumask_var(&watchdog_cpumask_for_smpboot, GFP_KERNEL);
>
> alloc_cpumask_var could fail?

Good catch; if I get a failure I'll just return early without trying to
start the watchdog, since clearly things are too memory-constrained
to enable that functionality anyway.

Thanks!

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