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

[Chris Metcalf]

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

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