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Date: Tue, 13 Oct 2015 22:05:20 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <Waiman.Long@hpe.com>
Cc: Ingo Molnar <mingo@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, Scott J Norton <scott.norton@hpe.com>,
        Douglas Hatch <doug.hatch@hpe.com>,
        Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [PATCH v7 3/5] locking/pvqspinlock: Collect slowpath lock
 statistics
Message-ID: <20151013200520.GZ3816@twins.programming.kicks-ass.net>
References: <1442955044-43895-1-git-send-email-Waiman.Long@hpe.com>
 <1442955044-43895-4-git-send-email-Waiman.Long@hpe.com>
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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?