Re: [PATCH v5 1/8] qspinlock: Introducing a 4-byte queue spinlock implementation

[Waiman Long <waiman.long@hp.com>]

On 02/26/2014 11:22 AM, Peter Zijlstra wrote:
> On Wed, Feb 26, 2014 at 10:14:21AM -0500, Waiman Long wrote:
>
>> +struct qnode {
>> +	u32		 wait;		/* Waiting flag		*/
>> +	struct qnode	*next;		/* Next queue node addr */
>> +};
>> +
>> +struct qnode_set {
>> +	struct qnode	nodes[MAX_QNODES];
>> +	int		node_idx;	/* Current node to use */
>> +};
>> +
>> +/*
>> + * Per-CPU queue node structures
>> + */
>> +static DEFINE_PER_CPU_ALIGNED(struct qnode_set, qnset) = { {{0}}, 0 };
> So I've not yet wrapped my head around any of this; and I see a later
> patch adds some paravirt gunk to this, but it does blow you can't keep
> it a single cacheline for the sane case.

There is a 4-byte hole in the qnode structure for x86-64. I did try to 
make the additional PV fields used only 4 bytes so that there is no 
increase in size in the qnode structure unless we need to support 16K 
CPUs or more.

-Longman