Re: [PATCH v7 3/5] locking/pvqspinlock: Collect slowpath lock statistics

[Waiman Long]

On 10/13/2015 04:05 PM, Peter Zijlstra wrote:
> On Tue, Sep 22, 2015 at 04:50:42PM -0400, Waiman Long wrote:
>
> > @@ -100,10 +242,13 @@ static struct qspinlock **pv_hash(struct qspinlock *lock, struct pv_node *node)
> >   {
> >   	unsigned long offset, hash = hash_ptr(lock, pv_lock_hash_bits);
> >   	struct pv_hash_entry *he;
> > +	int hopcnt = 0;
> >
> >   	for_each_hash_entry(he, offset, hash) {
> > +		hopcnt++;
> >   		if (!cmpxchg(&he->lock, NULL, lock)) {
> >   			WRITE_ONCE(he->node, node);
> > +			pvstat_hop(hopcnt);
> >   			return&he->lock;
> >   		}
> >   	}
> > @@ -164,9 +309,10 @@ static void pv_init_node(struct mcs_spinlock *node)
> >   static void pv_wait_node(struct mcs_spinlock *node)
> >   {
> >   	struct pv_node *pn = (struct pv_node *)node;
> > +	int waitcnt = 0;
> >   	int loop;
> >
> > -	for (;;) {
> > +	for (;; waitcnt++) {
> >   		for (loop = SPIN_THRESHOLD; loop; loop--) {
> >   			if (READ_ONCE(node->locked))
> >   				return;
> > @@ -250,6 +401,7 @@ static void pv_wait_head(struct qspinlock *lock, struct mcs_spinlock *node)
> >   	struct pv_node *pn = (struct pv_node *)node;
> >   	struct __qspinlock *l = (void *)lock;
> >   	struct qspinlock **lp = NULL;
> > +	int waitcnt = 0;
> >   	int loop;
> >
> >   	/*
> > @@ -259,7 +411,7 @@ static void pv_wait_head(struct qspinlock *lock, struct mcs_spinlock *node)
> >   	if (READ_ONCE(pn->state) == vcpu_hashed)
> >   		lp = (struct qspinlock **)1;
> >
> > -	for (;;) {
> > +	for (;; waitcnt++) {
> >   		for (loop = SPIN_THRESHOLD; loop; loop--) {
> >   			if (!READ_ONCE(l->locked))
> >   				return;
> These things are ugly; did you verify that they compile away for the
> !stats case?

The waitcnt was added to track if a vCPU was suspended again without 
getting the lock after being kicked. I will double check if it will be 
compiled away in the !stat case.

Cheers,
Longman
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