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

[Waiman Long]

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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