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

[Waiman Long]

On 11/06/2015 10:01 AM, Peter Zijlstra wrote:
> On Fri, Oct 30, 2015 at 07:26:37PM -0400, Waiman Long wrote:
> > +++ b/kernel/locking/qspinlock_paravirt.h
> > @@ -23,6 +23,19 @@
> >   #define _Q_SLOW_VAL	(3U<<  _Q_LOCKED_OFFSET)
> >
> >   /*
> > + * Queue Node Adaptive Spinning
> > + *
> > + * A queue node vCPU will stop spinning if the vCPU in the previous node is
> > + * not running. The one lock stealing attempt allowed at slowpath entry
> > + * mitigates the slight slowdown for non-overcommitted guest with this
> > + * aggressive wait-early mechanism.
> > + *
> > + * The status of the previous node will be checked at fixed interval
> > + * controlled by PV_PREV_CHECK_MASK.
> > + */
> > +#define PV_PREV_CHECK_MASK	0xff
> > +
> > +/*
> >    * Queue node uses: vcpu_running&  vcpu_halted.
> >    * Queue head uses: vcpu_running&  vcpu_hashed.
> >    */
> > @@ -202,6 +215,20 @@ static struct pv_node *pv_unhash(struct qspinlock *lock)
> >   }
> >
> >   /*
> > + * Return true if when it is time to check the previous node which is not
> > + * in a running state.
> > + */
> > +static inline bool
> > +pv_wait_early(struct pv_node *prev, int loop)
> > +{
> > +
> > +	if ((loop&  PV_PREV_CHECK_MASK) != 0)
> > +		return false;
> > +
> > +	return READ_ONCE(prev->state) != vcpu_running;
> > +}
> So it appears to me the sole purpose of PV_PREV_CHECK_MASK it to avoid
> touching the prev->state cacheline too hard. Yet that is not mentioned
> anywhere above.

Yes, that is true. I will add a comment to that effect.

> > +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.

        for (;; waitcnt++) {
                /*
+                * Set correct vCPU state to be used by queue node 
wait-early
+                * mechanism.
+                */
+               WRITE_ONCE(pn->state, vcpu_running);
+
+               /*
                 * Set the pending bit in the active lock spinning loop to
                 * disable lock stealing. However, the pending bit check in
                 * pv_queued_spin_trylock_unfair() and the setting/clearing
@@ -374,6 +414,7 @@ static u32 pv_wait_head_lock(struct qspinlock *lock, 
struct mcs_spinlock *node)
                                goto gotlock;
                        }
                }
+               WRITE_ONCE(pn->state, vcpu_halted);

> FYI, I think I've now seen all patches ;-)

Thanks for the review. I will work on fixing the issues you identified 
and issue a new patch series next week.

Cheers,
Longman
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