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

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Date:	Tue, 13 Oct 2015 17:06:17 -0400
From:	Waiman Long <waiman.long@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v7 3/5] locking/pvqspinlock: Collect slowpath lock statistics

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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