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

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Date:	Mon, 2 Nov 2015 17:40:40 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@....com>
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 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.

> @@ -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 ?

> +/*
> + * 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.

> +		kicks     += per_cpu(qstats[qstat_pv_kick_unlock],  cpu);
> +		latencies += per_cpu(qstats[qstat_pv_latency_kick], cpu);
> +	}
> +
> +	/* Rounded to the nearest ns */
> +	return sprintf(buf, "%llu\n", kicks ? (latencies + kicks/2)/kicks : 0);
> +}
--
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