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

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Message-ID: <20170419083640.hpybu2lard6qnj5s@hirez.programming.kicks-ass.net>
Date:   Wed, 19 Apr 2017 10:36:40 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        John Stultz <john.stultz@...aro.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        linux-pm@...r.kernel.org, Arjan van de Ven <arjan@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Rik van Riel <riel@...hat.com>
Subject: Re: [patch V2 08/10] timer: Implement the hierarchical pull model

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.