Re: [patch V2 08/10] timer: Implement the hierarchical pull model

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Apr 19, 2017 at 10:31:08AM +0200, Thomas Gleixner wrote:
> On Wed, 19 Apr 2017, Peter Zijlstra wrote:
> > On Tue, Apr 18, 2017 at 01:11:10PM +0200, Thomas Gleixner wrote:

> > > +	}
> > > +	/* Allocate and set up a new group */
> > > +	group = kzalloc_node(sizeof(*group), GFP_KERNEL, node);
> > > +	if (!group)
> > > +		return ERR_PTR(-ENOMEM);
> > > +
> > > +	if (!zalloc_cpumask_var_node(&group->cpus, GFP_KERNEL, node)) {
> > > +		kfree(group);
> > > +		return ERR_PTR(-ENOMEM);
> > > +	}
> > 
> > So if you place that cpumask last, you can do:
> > 
> > 	group = kzalloc_node(sizeof(*group) + cpumask_size(),
> > 	                     GFP_KERNEL, node);
> 
> Hrm, that would allocate extra space for CPUMASK_OFF_STACK=n. I'll have a
> look.
> 

How so? Remember your structure will look like:

struct group {
	/* ... */
	unsigned long cpumask[0];
};

Which, even for the CPUMASK_OFF_STACK=n case, is exactly 0 bytes of
total storage.