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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