linux-kernel - Re: [PATCH tip/locking/core v9 4/6] locking/pvqspinlock: Collect slowpath lock statistics

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Date:	Thu, 05 Nov 2015 11:29:29 -0500
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 tip/locking/core v9 4/6] locking/pvqspinlock: Collect
 slowpath lock statistics

On 11/02/2015 11:40 AM, Peter Zijlstra wrote:
> On Fri, Oct 30, 2015 at 07:26:35PM -0400, Waiman Long wrote:
>> This patch enables the accumulation of kicking and waiting related
>> PV qspinlock statistics when the new QUEUED_LOCK_STAT configuration
>> option is selected. It also enables the collection of data which
>> enable us to calculate the kicking and wakeup latencies which have
>> a heavy dependency on the CPUs being used.
>>
>> The statistical counters are per-cpu variables to minimize the
>> performance overhead in their updates. These counters are exported
>> via the sysfs filesystem under the /sys/kernel/qlockstat directory.
>> When the corresponding sysfs files are read, summation and computing
>> of the required data are then performed.
> Why did you switch to sysfs? You can create custom debugfs files too.

I was not aware of that capability. So you mean using 
debugfs_create_file() using custom file_operations. Right? That doesn't 
seem to be easier than using sysfs. However, I can use that if you think 
it is better to use debugfs.

>
>> @@ -259,7 +275,7 @@ 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;
> Did you check that goes away when !STAT ?

Yes, the increment code goes away when !STAT. I had added a comment to 
talk about that.

>
>> +/*
>> + * Return the average kick latency (ns) = pv_latency_kick/pv_kick_unlock
>> + */
>> +static ssize_t
>> +kick_latency_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf)
>> +{
>> +	int cpu;
>> +	u64 latencies = 0, kicks = 0;
>> +
>> +	for_each_online_cpu(cpu) {
> I think you need for_each_possible_cpu(), otherwise the results will
> change with hotplug operations.

Right, I will make the change.

Cheers,
Longman
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