Re: [PATCH tip/locking/core v9 6/6] locking/pvqspinlock: Queue node adaptive spinning

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Nov 09, 2015 at 11:51:20AM -0500, Waiman Long wrote:
> On 11/06/2015 03:37 PM, Peter Zijlstra wrote:
> >On Fri, Nov 06, 2015 at 12:54:06PM -0500, Waiman Long wrote:
> >>>>+static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
> >>>>  {
> >>>>  	struct pv_node *pn = (struct pv_node *)node;
> >>>>+	struct pv_node *pp = (struct pv_node *)prev;
> >>>>  	int waitcnt = 0;
> >>>>  	int loop;
> >>>>+	bool wait_early;
> >>>>
> >>>>  	/* waitcnt processing will be compiled out if !QUEUED_LOCK_STAT */
> >>>>  	for (;; waitcnt++) {
> >>>>-		for (loop = SPIN_THRESHOLD; loop; loop--) {
> >>>>+		for (wait_early = false, loop = SPIN_THRESHOLD; loop; loop--) {
> >>>>  			if (READ_ONCE(node->locked))
> >>>>  				return;
> >>>>+			if (pv_wait_early(pp, loop)) {
> >>>>+				wait_early = true;
> >>>>+				break;
> >>>>+			}
> >>>>  			cpu_relax();
> >>>>  		}
> >>>>
> >>>So if prev points to another node, it will never see vcpu_running. Was
> >>>that fully intended?
> >>I had added code in pv_wait_head_or_lock to set the state appropriately for
> >>the queue head vCPU.
> >Yes, but that's the head, for nodes we'll always have halted or hashed.
> 
> The node state was initialized to be vcpu_running. In pv_wait_node(), it
> will be changed to vcpu_halted before sleeping and back to vcpu_running
> after that. So it is not true that it is either halted or hashed.

Durh,.. I mixed up pv_wait_node() and pv_wait_head() I think. Sorry for
the noise.