Re: [RFC][PATCH 05/11] perf: register pmu implementations

Peter Zijlstra
2010-06-28T15:16:56Z

On Mon, 2010-06-28 at 15:21 +0200, Frederic Weisbecker wrote:
> On Thu, Jun 24, 2010 at 04:28:09PM +0200, Peter Zijlstra wrote:
> > + if (bp->attr.type != PERF_TYPE_BREAKPOINT)
> > + return -ENOENT;
> > +
> > + err = register_perf_hw_breakpoint(bp);
> > + if (err)
> > + return err;
> > +
> > + bp->destroy = bp_perf_event_destroy;

> Seems it would make sense to also have destroy in the pmu, it's the same
> along every events in the same class right?
>
> But this can be for later.

Ah, indeed.

> > +static LIST_HEAD(pmus);
> > +static DEFINE_MUTEX(pmus_lock);
> > +static struct srcu_struct pmus_srcu;
> > +
> > +int perf_pmu_register(struct pmu *pmu)
> > +{
> > + mutex_lock(&pmus_lock);
> > + list_add_rcu(&pmu->entry, &pmus);
> > + mutex_unlock(&pmus_lock);
> > +
> > + return 0;
> > +}
> > +
> > +void perf_pmu_unregister(struct pmu *pmu)
> > +{
> > + mutex_lock(&pmus_lock);
> > + list_del_rcu(&pmu->entry);
> > + mutex_unlock(&pmus_lock);
> >
> > - atomic_inc(&perf_swevent_enabled[event_id]);
> > - event->destroy = sw_perf_event_destroy;
> > + synchronize_srcu(&pmus_srcu);
> > +}
> > +
> > +struct pmu *perf_init_event(struct perf_event *event)
> > +{
> > + struct pmu *pmu = NULL;
> > + int idx;
> > +
> > + idx = srcu_read_lock(&pmus_srcu);
> > + list_for_each_entry_rcu(pmu, &pmus, entry) {
> > + int ret = pmu->event_init(event);
> > + if (!ret)
> > + break;
> > + if (ret != -ENOENT) {
> > + pmu = ERR_PTR(ret);
> > + break;
> > }
> > - pmu = &perf_ops_generic;
> > - break;
> > }
> > + srcu_read_unlock(&pmus_srcu, idx);
> >
> > return pmu;
> > }
>
>
>
> I'm still not sure why all this locking is needed. We don't even
> support pmus in modules.
>
> Is there something coming soon that will use this?
> I remember something about KVM.

Possibly, not sure. We could put the unregister thing in a later patch,
but I wanted to make sure it was sanely possibly and its only a few
lines of code.

> And who will have to use srcu? It seems the event fastpath would
> be concerned, right? Will that have an impact on the performances?

Only event creation like above (perf_init_event) will have to use SRCU,
so not really a hot path.

> > @@ -5743,15 +5742,15 @@ perf_cpu_notify(struct notifier_block *s
> > {
> > unsigned int cpu = (long)hcpu;
> >
> > - switch (action) {
> > + switch (action & ~CPU_TASKS_FROZEN) {
> >
> > case CPU_UP_PREPARE:
> > - case CPU_UP_PREPARE_FROZEN:
> > + case CPU_DOWN_FAILED:
> > perf_event_init_cpu(cpu);
> > break;
> >
> > + case CPU_UP_CANCELED:
> > case CPU_DOWN_PREPARE:
> > - case CPU_DOWN_PREPARE_FROZEN:
> > perf_event_exit_cpu(cpu);
> > break;
>
>
>
> That doesn't seem to be related to this patch initial topic.

Ah indeed, that needs to go live in its own patch.