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Date: Wed, 19 Apr 2017 11:03:05 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Peter Zijlstra <peterz@infradead.org>
cc: LKML <linux-kernel@vger.kernel.org>,
        John Stultz <john.stultz@linaro.org>,
        Eric Dumazet <edumazet@google.com>,
        Anna-Maria Gleixner <anna-maria@linutronix.de>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        linux-pm@vger.kernel.org, Arjan van de Ven <arjan@infradead.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Rik van Riel <riel@redhat.com>
Subject: Re: [patch V2 08/10] timer: Implement the hierarchical pull model
In-Reply-To: <20170419083640.hpybu2lard6qnj5s@hirez.programming.kicks-ass.net>
Message-ID: <alpine.DEB.2.20.1704191100340.1829@nanos>
References: <20170418111102.490432548@linutronix.de> <20170418111401.016420305@linutronix.de> <20170419081116.GA3029@worktop.programming.kicks-ass.net> <alpine.DEB.2.20.1704191028230.1829@nanos>
 <20170419083640.hpybu2lard6qnj5s@hirez.programming.kicks-ass.net>
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On Wed, 19 Apr 2017, Peter Zijlstra wrote:
> 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.

-ENOTENOUGHCOFFEE